Frequency,spectrum, and functional significance of <Emphasis Type="Italic">TP53</Emphasis> mutations in patients with diffuse large B-cell lymphoma

A comparative analysis of oncogene mutations shows that variations in their frequency, spectrum, and hot-spot locations depends on the type of tumor and the ethnic origin of the population studied. The current version of the IARC TP53 Mutation Database lacks information about the frequency and spectrum of TP53 mutations in patients with DLBCL in Russia. The aim of this study was to assess the frequency and functional significance of TP53 mutations in patients with DLBCL in Novosibirsk. The TP53 coding sequence and the adjacent intron regions were analyzed by direct sequencing in the tumor material from 74 patients with DLBCL. Mutations of the TP53 coding sequence were found in 18 (24.3%) patients. These data are consistent with the frequency of TP53 mutations observed in other studies. The spectrum of nucleotide substitutions found in DLBCL specimens corresponded to that described in the IARC TP53 Mutation Database. According to bioinformatic data and to reported experiments in vitro, most of the mutations detected result in the production of functionally inactive p53. Our results show that DLBCL progression is accompanied by the functional selection for mutations in TP53 exons 5–8.     

Association between activated K-ras and c-erbB-2 oncogenes with "high-risk" and "low-risk" human papilloma virus types in preinvasive cervical lesions

Clinical and epidemiological data have linked cervical cancer to the Human Papilloma Virus (HPV) infection. However, the presence of HPV infection alone is not enough to cause tumorigenesis, suggesting a role for additional host-cell genetic factors. The aim of the present work was to study the association of K-ras and c-erbB-2 mutations in cervical tissue samples with different grades of dysplasia and infected with HPV-6 ("low-risk" type) or HPV-16 and HPV-18 ("high-risk" types). Negative HPV-DNA samples were used as controls. The detection of K-ras and c-erbB-2 activation were performed by Artificial Refractory Mutation System (ARMS)-PCR and semiquantitative PCR, respectively. Statistical analysis showed a highly significant difference in K-ras codon 12 mutation frequency between high-risk and low-risk HPV-infected samples (p<0.05). On the other hand, amplification of the c-erbB-2 oncogene appeared associated to tissue samples infected with HPV-6 (p<0.003). Cervical carcinoma appears to arise from a series of well-characterized progressive histological changes, but the genetic alterations necessary for cervical tumorigenesis are not yet clear. These results raise the possibility for a role of certain proto-oncogenes and their activation in cervical neoplasia.     

Effects of oxygen radical scavengers and antioxidants on phagocyte-induced mutagenesis

Phagocytic leukocytes from normal humans can produce mutations in bacteria. To define further the role of oxygen radicals in this mutagenic process, we performed experiments in which scavengers or antioxidants were added to the incubation of phagocytes and bacteria. We found that 1) superoxide dismutase, catalase, mannitol, and benzoate were all capable of inhibiting mutation, 2) sulfhydryl compounds and vitamin E were also inhibitory, and 3) the presence of vitamin C in the incubations increased the mutation frequency. These data suggest an important role for hydroxyl radicals in mediating phagocyte-induced mutations.     

Suggestions for "safe" residue substitutions in site-directed mutagenesis

The conserved topological structure observed in various molecular families such as globins or cytochromes c allows structural equivalencing of residues in every homologous structure and defines in a coherent way a global alignment in each sequence family. A search was performed for equivalent residue pairs in various topological families that were buried in protein cores or exposed at the protein surface and that had mutated but maintained similar unmutated environments. Amino acid residues with atoms in contact with the mutated residue pairs defined the environment. Matrices of preferred amino acid exchanges were then constructed and preferred or avoided amino acid substitutions deduced. Given the conserved atomic neighborhoods, such natural in vivo substitutions are subject to similar constrains as point mutations performed in site-directed mutagenesis experiments. The exchange matrices should provide guidelines for "safe" amino acid substitutions least likely to disturb the protein structure, either locally or in its overall folding pathway, and most likely to allow probing the structural and functional significance of the substituted site.     

Characterization of Electrode Materials for Lithium Ion and Sodium Ion Batteries Using Synchrotron Radiation Techniques

Intercalation compounds such as transition metal oxides or phosphates are the most commonly used electrode materials in Li-ion and Na-ion batteries. During insertion or removal of alkali metal ions, the redox states of transition metals in the compounds change and structural transformations such as phase transitions and/or lattice parameter increases or decreases occur. These behaviors in turn determine important characteristics of the batteries such as the potential profiles, rate capabilities, and cycle lives. The extremely bright and tunable x-rays produced by synchrotron radiation allow rapid acquisition of high-resolution data that provide information about these processes. Transformations in the bulk materials, such as phase transitions, can be directly observed using X-ray diffraction (XRD), while X-ray absorption spectroscopy (XAS) gives information about the local electronic and geometric structures (e.g. changes in redox states and bond lengths). In situ experiments carried out on operating cells are particularly useful because they allow direct correlation between the electrochemical and structural properties of the materials. These experiments are time-consuming and can be challenging to design due to the reactivity and air-sensitivity of the alkali metal anodes used in the half-cell configurations, and/or the possibility of signal interference from other cell components and hardware. For these reasons, it is appropriate to carry out ex situ experiments (e.g. on electrodes harvested from partially charged or cycled cells) in some cases. Here, we present detailed protocols for the preparation of both ex situ and in situ samples for experiments involving synchrotron radiation and demonstrate how these experiments are done.     

"Wide-open" 1.3 A structure of a multidrug-resistant HIV-1 protease as a drug target

This report examines structural changes in a highly mutated, clinical multidrug-resistant HIV-1 protease, and the crystal structure has been solved to 1.3 A resolution in the absence of any inhibitor. This protease variant contains codon mutations at positions 10, 36, 46, 54, 62, 63, 71, 82, 84, and 90 that confer resistance to protease inhibitors. Major differences between the wild-type and the variant include a structural change initiated by the M36V mutation and amplified by additional mutations in the flaps of the protease, resulting in a "wide-open" structure that represents an opening that is 8 A wider than the "open" structure of the wild-type protease. A second structural change is triggered by the L90M mutation that results in reshaping the 23-32 segment. A third key structural change of the protease is due to the mutations from longer to shorter amino acid side chains at positions 82 and 84.     

Identifying and quantifying sources of variation in microarray data using high-density cDNA membrane arrays.

Microarray experiments involve many steps, including spotting cDNA, extracting RNA, labeling targets, hybridizing, scanning, and analyzing images. Each step introduces variability, confounding our ability to obtain accurate estimates of the biological differences between samples. We ran repeated experiments using high-density cDNA microarray membranes (Research Genetics Human GeneFilters Microarrays Version I) and 33P-labeled targets. Total RNA was extracted from a Burkitt lymphoma cell line (GA-10). We estimated the components of variation coming from: (1) image analysis, (2) exposure time to PhosphorImager screens, (3) differences in membranes, (4) reuse of membranes, and (5) differences in targets prepared from two independent RNA extractions. Variation was assessed qualitatively using a clustering algorithm and quantitatively using a version of ANOVA adapted to multivariate microarray data. The largest contribution to variation came from reusing membranes, which contributed 38% of the total variation. Differences in membranes and in exposure time each contributed about 10%. Differences in target preparations contributed less than 5%. The effect of image quantification was negligible. Much of the effect from reusing membranes was attributable to increasing levels of background radiation and can be reduced by using membranes at most four times. The effects of exposure time, which were partly attributable to variation in the scanning process, can be minimized by using the same exposure time for all experiments.     

Maintenance of a "Parasitic" B Chromosome in the Grasshopper MELANOPLUS FEMUR-RUBRUM

About 10-15% of the males and females of the grasshopper Melanoplus femur-rubrum collected near Rochester, New York, possessed a supernumerary B chromosome. The frequency of the B chromosome remained fairly constant during the years 1971-1974. The B chromosome was shown previously to be transmitted at a rate of about 0.5 and 0.8 by 1B males and females, respectively. This study was designed to determine the forces preventing the B chromosome from increasing in frequency due to the high rate of transmission by the females. Eighty inseminated females collected in the wild were analyzed cytologically together with their embryos (10-20 per female). Ten of the 80 females had a B chromosome, and they transmitted it at a rate of about 0.75. Among the 983 embryos analyzed, 0.141 had one B, 0.007 had two, and the mean number of B chromosomes per embryo was 0.155. The frequency of the B chromosome in the sperm pool (0.061) was consistent with a 0.5 rate of transmission. Individuals with two B chromosomes apparently have low viability, because about six were expected, but none was found among 851 adult males and females examined. The data suggest that the viability of the 1B individuals was only about 0.86 that of the OB individuals. There was no evidence that the B chromosome increased the fecundity of either the 1B males or females. It was concluded, therefore, that the B chromosome reduced the fitness of all the individuals carrying it and was thus "parasitic," and that it was maintained in the population only because of its high transmission rate. The maintenance of other B chromosomes with high transmission rates is reviewed.     

Sequence requirements in the catalytic core of the "10-23" DNA enzyme

A systematic mutagenesis study of the "10-23" DNA enzyme was performed to analyze the sequence requirements of its catalytic domain. Therefore, each of the 15 core nucleotides was substituted separately by the remaining three naturally occurring nucleotides. Changes at the borders of the catalytic domain led to a dramatic loss of enzymatic activity, whereas several nucleotides in between could be exchanged without severe effects. Thymidine at position 8 had the lowest degree of conservation and its substitution by any of the other three nucleotides caused only a minor loss of activity. In addition to the standard nucleotides (adenosine, guanosine, thymidine, or cytidine) modified nucleotides were used to gain further information about the role of individual functional groups. Again, thymidine at position 8 as well as some other nucleotides could be substituted by inosine without severe effects on the catalytic activity. For two positions, additional experiments with 2-aminopurine and deoxypurine, respectively, were performed to obtain information about the specific role of functional groups. In addition to sequence-function relationships of the DNA enzyme, this study provides information about suitable sites to introduce modified nucleotides for further functional studies or for internal stabilization of the DNA enzyme against endonucleolytic attack.     

Genetic control of purine biosynthesis in Pichia methanolica yeast. The ADE5 (PUR4) gene, controlling 5'-phosphoribosylformyl glycineamide synthetase

By comparing published and experimental data on spontaneous mutability of early genes controlling biosynthesis of purine nucleotides (BPN) in different yeast species in the system "from red to white," it was shown that the PUR4 gene encoding 5'-phosphoribosylformyl glycinamidine synthetase (FGAM-synthetase) (EC 6.3.5.3) is the most mutable gene in yeast Saccharomyces cerevisiae (the ADE6 gene), Schizosaccharomyces pombe (the ade3 gene), and Pichia methanolica (the ADE5 gene). This correlates with a considerably large size of the FGAM-synthetase polypeptide, as compared to the products of other genes belonging to this group. Study of characteristics of spontaneous mutations in early BPN genes of P. methanolica demonstrated that the vast majority of unstable ade5sU alleles (mutations with a high reversion frequency ranging from 0.2 x 10(-6) to 2 x 10(-6)) appeared solely among mutants for the ADE5 gene. Based on these results, it was assumed that there are two independent mechanisms responsible for reversions of spontaneous mutations in this gene. The DNA sequence that can compensate for the P. methanolica ade5 mutation and probably is the structural P-ADE5 gene, was cloned from a genomic library of P. methanolica by the ade6 mutation complementation in the recipient S. cerevisiae strain.     

The effect of dose rate on the frequency of specific-locus mutations induced in mouse spermatogonia is restricted to larger lesions; a retrospective analysis of historical data

A series of 19 large-scale germ-cell mutagenesis experiments conducted several decades ago led to the conclusion that low-LET radiation delivered to mouse spermatogonia at dose rates of 0.8 R/min and below induced only about one-third as many specific-locus mutations as did single, acute exposures at 24 R/min and above. A two-hit origin of the mutations was deemed unlikely in view of the then prevailing evidence for the small size of genetic lesions in spermatogonia. Instead, the dose-rate effect was hypothesized to be the result of a repair system that exists in spermatogonia, but not in more mature male reproductive cells. More recent genetic and molecular studies on the marker genes have identified the phenotypes associated with specific states of the mutant chromosomes, and it is now possible retrospectively to classify individual past mutations as "large lesions" or "other lesions". The mutation-frequency difference between high and low dose rates is restricted to the large lesion mutations, for which the dose-curve slopes differ by a factor exceeding 3.4. For other lesion mutations, there is essentially no difference between the slopes for protracted and acute irradiations; induced other lesions frequencies per unit dose remain similar for dose rates ranging over more than 7 orders of magnitude. For large lesions, these values rise sharply at dose rates >0.8 R/min, though they remain similar within the whole range of protracted doses, failing to provide evidence for a threshold dose rate. The downward bend at high doses that had been noted for X-ray-induced specific-locus mutations as a whole and ascribed to a positive correlation between spermatogonial death and mutation load is now found to be restricted to large lesion mutations. There is a marked difference between the mutation spectra (distributions among the seven loci) for large lesions and other lesions. Within each class, however, the spectra are similar for acute and protracted irradiation.     

Clinical and genetic profile of patients with medullary thyroid cancer treated in the Cancer Centre--Institute of Oncology in Warsaw

INTRODUCTION: The aim of this study was to analyse the distribution and frequency of mutations and their correlations with clinical phenotypes of patients with MTC, to reveal the differences between sporadic and familial type of MTC, and to describe the phenotypes of patients. MATERIALS AND METHODS: 212 patients with medullary thyroid cancer (MTC) were treated in Cancer Centre in Warsaw between 1997 and 2005. In most patients, DNA isolated from peripheral blood leukocytes was tested for RET gene mutations by sequencing and accordingly MTC form was assessed. Genetic testing was performed in the relatives of patients with familial MTC in order to distinguish asymptomatic mutation carriers from noncarriers. RESULTS: RET gene mutations were identified in 46 patients (22%). The others were found noncarriers and sporadic MTC was diagnosed. MEN 2A/FMTC syndrome (multiple endocrine neoplasia type 2A/ familial type of MTC) was diagnosed in 44 patients, MEN 2B syndrome (multiple endocrine neoplasia type 2B) in 2 patients. In patients with sporadic and familial MTC, age at diagnosis and multifocal occurrence was analysed, and the results were found to be in accordance with those of other research centres. However, the distribution and frequency of mutations, as well as some clinical data, such as the frequency of pheochromocytoma occurrence as the first manifestation of MEN syndrome, differed from the published data, and further studies are necessary to reveal the reasons of these differences. CONCLUSIONS: DNA testing for RET gene mutations is reliable as a diagnostic tool and therefore it should be performed for screening of all patients with MTC or other diseases of MEN syndrome.     

Experiments in microbial evolution: new enzymes, new metabolic activities

Biological evolution has resulted in a richness and diversity of species. Among microorganisms this is most evident in the wealth and diversity of biochemical transformations. Evidence for evolutionary relationships may be obtained from comparative studies, but with microorganisms it is also possible to follow evolution in action. Microbial populations adapt rapidly to changes in the environment and the evolution of new metabolic activities can be observed in laboratory experiments. The enzymes of many catabolic pathways are synthesized in response to the presence of inducing substrates. New catabolic activities may be acquired by mutations in regulatory genes resulting in alterations in the specificity of induction, or in enzyme synthesis in the absence of inducer. Mutations in structural genes may given rise to enzymes with altered substrate specificities. In bacteria, catabolic genes may be carried on plasmids and the exchange of plasmids among bacterial populations increases the evolutionary potential. Experiments in microbial evolution have produced strains with novel catabolic activities involving regulatory or structural gene mutations, gene duplications and plasmid exchange. Enzymes studied in this way include amidase, ribitol dehydrogenase, evolved beta-galactosidase, and enzymes of the catabolic pathways for pentoses and pentitols and haloaromatic compounds.     

X-ray-induced mutations affecting the level of the enzyme alcohol dehydrogenase in Drosophila melanogaster: frequency and genetic analysis of null-enzyme mutants.

The frequency of X-ray-induced (null-enzyme) mutations at the alcohol dehydrogenase locus in Drosophila melanogaster was measured. The rate of recovery of chromosomes that fail to direct the synthesis of a functional Adh protein is 3 x 10(-8) per R for chromosomes that do not include large chromosome rearrangements. However, this analysis excludes a larger number of chromosomes that are "null-enzyme mutations" because thye are deleted for the region of the Adh locus. The dose of X-rays required to induce a frequency of non-deletion null-enzyme mutants equal to the spontaneous frequency is about 73 rad calculated from the data reported in this communication.     

Frequency of rare electrophoretic protein variants in normal human beings and in congenital pathology

17 blood proteins of infants with rough and multiple congenital malformations (CM), prematurely born infants and sick newborns without developmental anomalies were studied electrophoretically in polyacrylamide and starch gels (62422 locus tests). The control included blood samples of healthy newborns from ordinary maternity hospitals (60234 locus tests). The frequency of rare protein variants in all the cases was higher in sick children than in healthy ones. The frequency of rare genes (corrected for electrophoretically "silent" alleles) was 2.16 X 10(-3) in infants with CM and 0.99 X 10(-4) in the control. Examination of parents of 11 congenitally malformed infants with rare protein variants showed that at least in 5 cases such variants were absent in the parents and might be attributed to "fresh" mutations. However, only 3 variants (1 for serum albumin and 2 for red cell esterase) represented rare heterozygotes with codominant expression. This corresponds to the frequency of 0.59 X 10(-3). In the total population of newborns the proportion of infants with CM was 0.02, which means that the population mutation rate is 1.18 X 10(-5) per gene per generation. The data obtained support the conclusion about strong pressure of stabilizing selection against de novo mutations which change electrophoretic mobility of the protein molecule. The reasons for discrepancy between our data and the recent results of Neel and Mohrenweiser (1984) are discussed.     

Comparison of the spectra of genetic damage in formaldehyde-induced ad-3 mutations between DNA repair-proficient and -deficient heterokaryons of Neurospora crassa.

The mutagenic effects of formaldehyde (FA) have been compared in DNA repair-proficient (heterokaryon 12) and DNA repair-deficient (heterokaryon 59) two-component heterokaryons of Neurospora crassa. The data from forward-mutation experiments were used to compare the spectra of FA-induced specific-locus mutations at two closely linked loci in the adenine-3 (ad-3) region and on the FA-induced inactivation of heterokaryotic conidia. Previous studies have demonstrated that specific-locus mutations at these two loci result from five major genotypic classes, namely two classes of gene/point mutations (ad-3A(R) and ad-3B(R)), and three classes of multilocus deletion mutations ([ad-3A](IR), [ad-3B](IR), and [ad-3A ad-3B](IR)). Genetic analysis of ad-3 mutants recovered from both heterokaryons after FA treatment demonstrates that predominantly gene/point mutations were found in H-12 (93.2% ad-3(R), 6.8% [ad-3](IR)) and a significantly higher frequency of multilocus deletion mutations in H-59 (62.8% ad-3(R), 37.0% [ad-3](IR)). The data from our experiments with FA on H-12 demonstrate and confirm the data from other assays that FA is a weak mutagen in this DNA repair-proficient strain. However, the data from our experiments with the DNA repair-deficient strain H-59 demonstrate that comparable concentrations of FA cause more pronounced inactivation of heterokaryotic conidia and, at the highest concentration tested, about a 35-fold higher frequency of ad-3 mutations. In addition, FA induced a 5.4-fold higher frequency of ad-3 mutations resulting from multilocus deletion mutation in H-59 than in H-12. Based on our earlier studies with X-ray-induced multilocus deletion mutations, it is this class of FA-induced ad-3 mutations that might be most expected to show deleterious heterozygous effects. The implications of the present data base from our experiments with Neurospora are that the mutagenic (and possibly the carcinogenic) effect of FA exposure might well vary in different human population subgroups.     

Genetic Control of Purine Biosynthesis in Yeast Pichia methanolica. The ADE5 (PUR4) Gene Controlling 5"-Phosphoribosylformyl Glycinamidine Synthetase

By comparing published and experimental data on spontaneous mutability of early genes controlling biosynthesis of purine nucleotides (BPN) in different yeast species in the system from red to white, it was shown that the PUR4 gene encoding 5"-phosphoribosylformyl glycinamidine synthetase (FGAM-synthetase) (EC 6.3.5.3) is the most mutable gene in yeastSaccharomyces cerevisiae (the ADE6 gene), Schizosaccharomyces pombe(the ade3 gene), andPichia methanolica (theADE5 gene). This correlates with a considerably large size of the FGAM-synthetase polypeptide, as compared to the products of other genes belonging to this group. Study of characteristics of spontaneous mutations in early BPN genes of P. methanolica demonstrated that the vast majority of unstable ade5s ^U alleles (mutations with a high reversion frequency ranging from 0.2 × 10^–6 to 2 × 10^–6) appeared solely among mutants for the ADE5 gene. Based on these results, it was assumed that there are two independent mechanisms responsible for reversions of spontaneous mutations in this gene. The DNA sequence that can compensate for theP. methanolica ade5mutation and probably is the structural P-ADE5gene, was cloned from a genomic library of P. methanolicaby the ade6 mutation complementation in the recipient S. cerevisiae strain.     

The approach to the Michaelis complex in lactate dehydrogenase: the substrate binding pathway

We examine here the dynamics of forming the Michaelis complex of the enzyme lactate dehydrogenase by characterizing the binding kinetics and thermodynamics of oxamate (a substrate mimic) to the binary lactate dehydrogenase/NADH complex over multiple timescales, from nanoseconds to tens of milliseconds. To access such a wide time range, we employ standard stopped-flow kinetic approaches (slower than 1 ms) and laser-induced temperature-jump relaxation spectroscopy (10 ns-10 ms). The emission from the nicotinamide ring of NADH is used as a marker of structural transformations. The results are well explained by a kinetic model that has binding taking place via a sequence of steps: the formation of an encounter complex in a bimolecular step followed by two unimolecular transformations on the microsecond/millisecond timescales. All steps are well described by single exponential kinetics. It appears that the various key components of the catalytically competent architecture are brought together as separate events, with the formation of strong hydrogen bonding between active site His(195) and substrate early in binding and the closure of the catalytically necessary protein surface loop over the bound substrate as the final event of the binding process. This loop remains closed during the entire period that chemistry takes place for native substrates; however, motions of other key molecular groups bringing the complex in and out of catalytic competence appear to occur on faster timescales. The on-enzyme K(d) values (the ratios of the microscopic rate constants for each unimolecular step) are not far from one. Either substantial, approximately 10-15%, transient melting of the protein or rearrangements of hydrogen bonding and solvent interactions of a number of water molecules or both appear to take place to permit substrate access to the protein binding site. The nature of activating the various steps in the binding process seems to be one overall involving substantial entropic changes.     

Independence of the processes of transposition and genome rearrangements induced by transposons

The data on the influence of the tnm mutations affecting transposition process on the deletion formation promoted by Tn and IS elements are presented. It was shown that the tnm mutations did not affect the frequency of deletion formation. The results of genetic analysis of the tnm mutant deficient in both transposition and genomic rearrangements induced by Tn9 inserted into lambda prophage, indicated that the mutant phenotype was caused by two different but linked mutations. A mutation affecting the process of genomic rearrangements was designated gerA2. The gerA2 mutation decreased sharply the frequency of rearrangements promoted by Tn9, Tn10 or Tn601 inserted into lambda prophage. However, this mutation had no influence upon transposition of the same Tn elements. The data obtained could be interpreted as indicating the independence of the processes of transposition and genomic rearrangements or as indication of the existence of specific steps of these processes.     

Is there a proportionality between the spontaneous and the X-ray induction rates of mutations?: Experiments with mutations at 13 X-chromosome loci in Drosophila melanogaster

The X-ray induction of recessive visible specific locus mutations at 14 X-chromsome loci was studied in Drosophila melanogaster using the "Maxy" technique. The X-ray exposure was 3000 R to 5-day-old males and the sampling of germ cells was restricted to mature spermatozoa. Presumptive mutant females recovered in the F1 generation were tested for transmission, allelism, fertility and viability in males. A total of 128 mutations (115 completes and 13 mosaics including those that were male viable as well as male-lethal) recovered among 38 898 female progeny were found to be transmitted. On the basis of the above frequency, the average mutation rate can be estimated as 7.8 X 10(-8)/locus/R; for mutations that were viable and fertile in males, the rate is 3.0 X 10(-5)/locus/R (49 mutations among 38 898 progeny). The frequency of mutations at the different loci encompassed a wide range: while no mutations were recovered at the raspberry and carnation loci, at others, the numbers ranged from 1 at echinus to 31 at garnet; in addition, the proportion of mutations that was male-viable was also different, depending on the locus. Schalet's extensive data on spontaneous mutations at 13 (of the 14 loci employed in the present study) loci permit an estimate of the spontaneous rate which is 6.1 X 10(-6)/locus (a total of39 mutations among 490 000 progeny); for mutations that were viable and fertile in males, the rate is 3.0 X 10(-6)/locus (19 mutations among 490 000 progeny). The mutability of the different loci varied over a 9-fold range. When the different loci are ranked depending on their relative mutability (for spontaneous and induced mutations) it is found that in general, loci that mutate spontaneously relatively more frequently are also those at which more mutations have been recovered in the radiation experiments and likewise, those that are less mutable spontaneously are also those that mutate less after irradiation. Since the data are limited, it is concluded that the above finding is not inconsistent with the assumption of proportionality between spontaneous and induction rates of mutations. On the basis of the above results, a doubling dose of 100 R can be calculated for the X-ray induction of specific-locus mutations in Drosophila spermatozoa.