Mutation Induction in the Male Recombination Strains of DROSOPHILA MELANOGASTER

One group of the second chromosome lines isolated from a southern Texas population of Drosophila melanogaster, which has been known to show relatively high frequencies of male recombinations, was found to increase the frequency of sex-linked recessive lethal mutations from a control frequency of 0.18% to 1.63%. The second group, which showed a very much reduced frequency of male recombinations, was found to cause a slight increase to 0.48%, although it was not statistically significant. The first group was also tested for the recessive lethal mutation frequency in the second chromosome; the frequency increased from a control frequency of 0.28% to 2.82%. Mapping of a portion of the sex-linked lethals indicated a distribution along the entire X chromosome, although there was a tendency of clustering towards the tip of the X chromosome. One sex-linked lethal line so far tested was found to be associated with an inversion (approximate breakpoints, 14A-18A). It was suggested that the element causing male recombination might be similar to the hi mutator gene studied earlier by Ives (1950).     

Radioprotective effect of six chemicals against X-ray-induced genetic damage in D. melanogaster

In Drosophila melanogater six chemicals were tested for radioprotectiveeffect against X-ray-induced genetic damage such as sex-linked recessive lethals and autosomal translocations using Oster's ring-X chromosome stock. A 2-day brood pattern was followed to score the damage induced at different spermatogenic stages separately. In all cases the chemicals were injected before X-irradiation. 10-mM solution of reduced glutathione (GSH) provided statistically significant protection against sex-linked recessive lethals in all broods. In translocation tests this chemical reduced the frequency in all broods but the result is not statistically significant. Cysteamine (MEA) did not show any protective effect but the frequency of lethals was slightly reduced in the first and fourth broods. 2-Aminoethyl isothiuronium Br·HBr (AET) showed a statistically significant protective effect when the data of the replicate experiments were pooled. Negative results were obtained for 5-hydroxytryptamine (5-HT) in sex-linked lethal tests. Aminoethyl phosphorothioate (AEPT) reduced the frequencies of both sex-linked lethals and autosomal translocations in all broods consistently but the results are not statistically significant. In tests for both lethals and translocations the reduction was largest in the stages with highest radiosensitivity. N(3-Aminopropyl)aminoethyl phosphorothioate (3AP-AEPT) gave no protection.     

Induction and persistence of micronuclei, sister-chromatid exchanges and chromosomal aberrations in splenocytes and bone-marrow cells of rats exposed to ethylene oxide

Studies on the induction and persistence of ethylene oxide (EO) induced chromosomal alterations in rat bone-marrow cells and splenocytes following in vivo exposure were carried out. Rats were exposed to ethylene oxide either chronically by inhalation (50-200ppm, 4 weeks, 5 days/week, 6h/day) or acutely by intraperitoneal injection (i.p.) at dose levels of 50-100ppm.Spontaneous- and induced-frequencies of micronuclei (MN), sister-chromatid exchanges (SCEs) and chromosomal aberrations were determined in rat bone-marrow cells, and in splenocytes following in vitro mitogen stimulation. Unstable chromosomal aberrations were studied in whole genome using standard Giemsa staining technique and fluorescence in situ hybridisation using probe for chromosome #2 was employed to detect chromosome translocations.Following chronic exposure, the cytogenetic analyses were carried out at days 5 and 21 in rat splenocytes, to study the induction and persistence of sister-chromatid exchanges. Following chronic exposure, ethylene oxide was effective in inducing SCEs, and markedly cells with high frequency SCEs were observed and they in-part persisted until day 21 post-exposure. However, no significant effect was observed in rat splenocytes for induction of MN and chromosomal aberrations. Following acute exposure, both SCEs and MN were increased significantly in rat bone-marrow cells as well as splenocytes.In conclusion, this study indicates that ethylene oxide at the concentrations employed by intraperitoneal injection or inhalation in adult rats is mutagenic and can induce both SCEs and MN.     

The influence of electrostatic and magnetic fields on mutation in Drosophila melanogaster spermatozoa.

Canton-S Drosophila melanogaster males were exposed to electrostatic and magnetic fields for 24 h to determine the influence of low energy fields on the production of sex-linked recessive lethal mutations in the mature, motile sperm. To detect sex-linked recessive lethal production in mature sperm the standard Muller-5 test was done. Exposure of the males to the magnetic field or the electrostatic field did not significantly affect the mutation frequency in mature sperm.     

Mutagenic activity of propylene oxide in bacterial and mammalian systems.

Propylene oxide is used extensively in the chemical and food manufacturing industries, but relatively little is known of its ability to interact with genetic material. Studies were undertaken to investigate its ability to induce gene mutations and primary DNA damage in bacteria and chromosomal damage in mammalian cells. The induction of base-substitution mutations was demonstrated in spot tests with strains of Salmonella typhimurium and Escherichia coli at 700 micrograms/plate of propylene oxide; inclusion of a preparation of rat-liver microsomes and cofactors (S9 mix) was without significant effect on this response. A linear dose--response relationship was recorded in plate tests with S. typhimurium strains TA100 and TA1535 over the range 100--750 micrograms/plate. After addition to dividing lymphocytes in cultures established from human peripheral blood, propylene oxide caused dose-related chromosomal damage, detected at 1.85 and 9.25 micrograms/ml. Oral administration of propylene oxide at 2 x 100, 2 x 250 or 2 x 500 mg/kg to male mice produced no detectable increases in the incidence of micronucleated, polychromatic erythrocytes in bone marrow. A male mouse dominant lethal test employing oral doses of 50 or 250 mg/kg/day for 14 days gave no evidence of mutagenic action on sperm. Intraperitoneal injections of propylene oxide at 2 x 300 mg/kg induced increased numbers of micronucleated erythrocytes in mice, but lower doses given by this route had no such effect. Possible reasons for the contrasting findings in vitro and in vivo are discussed.     

Mutagenic activity of ethylene oxide and propylene oxide under XPG proficient and deficient conditions in relation to N-7-(2-hydroxyalkyl)guanine levels in Drosophila

Ethylene oxide (EO) and propylene oxide (PO) are direct acting mutagens with high Swain-Scott s-values, which indicate that they react preferentially with ring nitrogens in the DNA. We have previously described that in the X-linked recessive lethal (RL) assay in Drosophila postmeiotic male germ cells EO is, per unit exposure dose, 5-10 times more mutagenic than PO. Furthermore, at the higher dose range of EO tested, 62.5-1000 ppm, up to 20-fold enhanced mutation rates were measured in the absence of maternal nucleotide excision repair (NER) compared to repair proficient conditions. The lower dose range of EO tested, 2-7.8 ppm, still produced a small increased mutation rate but without a significant elevated effect when the NER system is being suppressed. The lowest dose of PO tested, 15.6 ppm, produced only in NER- condition an increased mutation rate. The aim of the present study was to compare the mutagenic effect of EO and PO in the RL assay under XPG proficient and deficient conditions with the formation of N-7-(2-hydroxyethyl)guanine (7-HEG) and N-7-(2-hydroxypropyl)guanine (7-HPG), respectively, the major DNA adducts formed. The formation of 7-HEG and 7-HPG was investigated in Drosophila males exposed to EO and PO as a measure of internal dose for exposures ranging from 2 to 1000 or 2000 ppm, respectively, for 24h. Analysis of 7-HEG and 7-HPG, using a highly sensitive 32P-postlabelling assay, showed a linear increase of adduct levels over the entire dose range. The non-linear dose-response relationship for mutations could therefore not be explained by a reduced inhalation or increased detoxification at higher exposure levels. In analogy with the four times higher reactivity of EO the level of N-7-guanine alkylation per ppm was for EO 3.5-fold higher than that for PO. Per unit N-7-guanine alkylation EO was found to be slightly more mutagenic than PO, whereas PO was the more potent clastogenic agent. While this research has not identified the DNA lesions that cause the increase in repair deficient flies, it supports the hypothesis that efficient error-free repair of some N-alkylation products can explain why these agents tend to be weakly genotoxic or even inactive in repair-competent (premeiotic) germ cells of the mouse and the Drosophila fly.     

Cytogenetic effects of inhaled ozone in man

Peripheral blood samples were collected from 30 normal male volunteers before and at various intervals after inhaling 0.4 ppm ozone for 4 h. Data from 4 of the subjects were excluded from the analysis because of missing data points. The blood samples were cultured for 48 h, slides made and stained with a uniform Giemsa stain, and 100 metaphase spreads per subject per treatment scored for chromosome aberrations. Cells with suspected aberrations were photographed, destained, restained with a banding procedure and rephotographed to identify the specific chromosomes and regions involved.Pre-exposure, immediate post-exposure, 3 days post-exposure, 2 weeks post-exposure and 4 weeks post-exposure means for the percentage of cells with 46 chromosomes were 93.0, 93.6, 91.7, 94.5 and 94.2, respectively; in the same order, the mean number of cells with chromatid and/or chromosome breaks per 100 cells was 0.96, 0.85, 1.00, 0.88 and 0.81 respectively, and for chromatid and/or chromosome gaps per 100 cells: 1.35, 0.96, 1.35, 0.81 and 0.77, respectively. The means for each of these parameters as well as the mean frequencies of complex aberrations are not statistically significantly different between blood sampling times. The distribution of aberrations by chromosome and light and dark bands is not significantly influenced by ozone exposure.These data indicate no apparent detectable human cytogenetic effect due to exposure to ozone under the conditions of this experiment.     

Mutagenicity of sumithion tested in Drosophila somatic and germ cells

Sumithion, a broad-spectrum insecticide, was tested for its mutagenicity in the Drosophila wing-spot test and sex-linked recessive lethal test. Strains carrying the recessive mutant markers mwh and flr3 in their third chromosomes, expressed phenotypically as multiple trichomes or thickened and misshapen wing hairs in the adult wings, were used in the wing-spot test. Larvae transheterozygous for these markers were exposed to the insecticide in instant food and the sex-linked recessive lethal test was performed by the standard technique using the Basc strain. The compound is mutagenic in the wing primordial cells and induces recombination at high doses. Further, the frequency of induction of sex-linked recessive lethals is significant only at high treatment doses.     

The genotoxicity of the anti-cancer drug mitoxantrone in somatic and germ cells of Drosophila melanogaster.

The novel antineoplastic drug mitoxantrone was studied for its genotoxic effects in Drosophila melanogaster. In male germ cells, the clinical preparation Novantrone, the dihydrochloride salt of mitoxantrone, did not induce sex-linked recessive lethal mutations in feeding and injection experiments with adult flies, although statistically the results were inconclusive rather than truly negative. However, the free base mitoxantrone was weakly, but significantly genotoxic in this test (0.14% lethals/mM exposure concentration); this is most probably the result of prolonged exposure. On the other hand, both forms of mitoxantrone assayed were clearly genotoxic in the somatic mutation and recombination test of the wing. This test assays the cells of the proliferating imaginal wing discs of larvae. Depending on the feeding method used, the overall clone induction frequency was in the range of about 2-6 x 10(-5) per cell and cell generation and per mM exposure dose. Correction of these frequencies according to mean clone size led to slightly higher estimates (by about 5-25% higher). Although the majority of the clone induction events are due to mitotic recombination, a significant proportion can be attributed to mutational events (gene and chromosome mutations). The genotoxicity of mitoxantrone seems to depend mainly on impaired DNA synthesis in cycling cells owing to the compound's ability to inhibit topoisomerase II by intercalation into DNA.     

Mutagenic effects of nitrogen dioxide combined with methylurea and ethylurea in Drosophila melanogaster

The standard sex-linked recessive lethal test was used to test whether NO2 induces lethal mutations in male germ cells of Drosophila in the presence or absence of alkylureas. Methylurea, ethylurea and NO2 alone did not enhance the mutation frequency significantly. However, highly significant enhancement in the mutation frequency was observed when adult flies were exposed to NO2 (150--280 ppm) for 3 h after ingestion of methylurea (0.1 M) or ethylurea (0.1 M) for 2 days. Oral administration of ethylnitrosourea and also of methylurea or ethylurea that had been exposed to NO2 in vitro were more effective in increasing the mutation frequency than methylurea or ethylurea combined in vivo with NO2. These results suggest that ingested alkylurea is converted in vivo by inhaled NO2 to highly mutagenic nitrosoalkylurea and/or other mutagens. No significant enhancement of the mutation frequency was observed when flies were fed on methylurea solution after they had been exposed to NO2.     

Low-temperature-modulated fruit ripening is independent of ethylene in 'Sanuki Gold' kiwifruit

Fruit ripening in response to treatments with propylene, 1-methycyclopropene (1-MCP), and low temperature was characterized in 'Sanuki Gold' kiwifruit, Actinidia chinensis Planch. Propylene treatment immediately induced rapid fruit softening, increased AC-PG (polygalacturonase) and AC-EXP (expansin) mRNA accumulation, and stimulated an increase in the soluble solid concentration (SSC) and a decrease in titratable acidity (TA). After 3?d exposure to propylene, ethylene production and AC-PL (pectate lyase) mRNA accumulation were observed. 1-MCP treatment after 24?h exposure to propylene eliminated AC-PG mRNA accumulation and suppressed continued changes in SSC and TA. Application of 1-MCP at the start of the treatment, followed by continuous propylene exposure, markedly delayed fruit softening, and the expression of the cell wall-modifying genes, and changes in the SSC and TA, indicating that kiwifruit become insensitive to ethylene at least for 3?d following 1-MCP exposure. Surprisingly, significant fruit softening, mRNA accumulation of AC-PG, AC-PL, and AC-EXP, and decreased TA were observed without ethylene production in intact fruit stored at low temperature for 1 month, but not in fruit stored at room temperature. Repeated 1-MCP treatments (twice a week) failed to inhibit the changes that occurred in low temperature storage. These observations indicate that low temperature modulates the ripening of kiwifruit in an ethylene-independent manner, suggesting that kiwifruit ripening is inducible by either ethylene or low temperature signals.     

Mutagenicity of hycanthone in Drosophila melanogaster

The schistosomicidal agent hycanthone was tested for mutagenicity in Drosophila melanogaster. The compound was administered either by injection into adult males or by larval feeding. The following types of genetic damage were measured:(1) complete and mosaic sex-linked recessive lethal mutations; (2) II–III translocations; and (3) dominant lethals.In postmeiotic germ cells, especially in late spermatids, a pronounced increase was found in the frequency of sex-linked recessive lethals, both completes and mosaics. By contrast, translocations and dominant lethals were not induced.     

Mutagenicity of Ascaris chymotrypsin inhibitor in germ cells of mice

Chymotrypsin inhibitor isolated from Ascaris suum (ACHI) was tested for the induction of dominant lethal mutations in male mice. Dominant lethal effects of ACHI for the main stages of germ cell development were analyzed by mating at specific time points after dosing. Two groups of adult BALB/c males received 24 or 40 mg per kilogram body weight (BW) per day intraperitoneal (IP) injection of ACHI in sterile phosphate-buffered saline (PBS) for five consecutive days (subacute exposure). Males from a third group were administered single IP injections of ACHI—60 mg/kg BW (acute exposure). The control group received concurrent injections of PBS for five successive days. After the last dose, each male was mated with two untreated females. For fractionated examination with regard to successive germ cell stages (spermatozoa, spermatids, spermatocytes, spermatogonia), every second week, two other untreated virgin females were placed with each male for mating. The uteri of the females were inspected on the 15th day of gestation, and preimplantation loss and postimplantation loss determined from dominant lethal parameters. Exposure of mice germ cells to ACHI did not impair mating activity of males. Fertility index was reduced (P < 0.05) only for females mated at the third week with males exposed to the highest dose of ACHI. In the females bred to ACHI-treated males, significant (P < 0.05) increase in preimplantation loss was observed at postinjection weeks 1 (reflecting exposure to spermatozoa after single treatment and to spermatozoa or late spermatids after subacute dosing) and 3 (reflecting exposure to mid and early spermatids for acute dosing and to mid and early spermatids or late spermatocytes following acute treatment), regardless of dose and length of exposure to the inhibitor. At the 60-mg/kg-BW group, a significant increase of this parameter was also noted at week 5 (reflecting exposure to early spermatocytes). During mating days 15–21, a significant (P < 0.05) increase in postimplantation loss and dominant lethal effects were observed for all doses of ACHI. Acute ACHI exposure 5 weeks prior to mating resulted in dominant lethal effects in early spermatocytes. These preliminary data suggest that ACHI induces dominant lethal mutations at postmeiotic and meiotic stages of spermatogenesis, but spermatids are the most sensitive cell stage to the effect of ACHI. These results show that ACHI may be one of the factors causing disturbances in spermatogenesis leading to a reduction of host reproductive success.     

Genotoxic effects of inhaled ethylene oxide, propylene oxide and butylene oxide on germ cells: sensitivity of genetic endpoints in relation to dose and repair status

We report here results on forward mutation induction (recessive lethal mutations, RL) in Drosophila spermatozoa and spermatids by the three 1,2-alkyl-epoxides ethylene oxide (EO), propylene oxide (PO) and butylene oxide (BO), at doses ranging from 47 to 24,000 ppm h for EO, 375 to 48,000 ppm h for PO, and 24,000 to 91,200 ppm h for BO. The results indicate for EO mutation induction at doses 500-fold below the LD₅₀. In crosses of mutagenized NER⁺ males with NER⁺ females, the 500-fold increase in EO dose from 47 ppm h to 24,000 ppm h resulted in no more than a 17-fold enhanced mutant frequency in spermatozoa. This flat dose–response relationship is primarily the result of efficient repair of EO-induced DNA adducts in the fertilized egg, as was evident from the up to 40-fold or 240-fold increased mutant frequencies above NER⁻ or NER⁺ background levels, respectively, in crosses with NER⁻ females. With decreasing dose, / ratios decreased from 9 to 14 at high doses down to ≈1 at the two lowest doses, indicating that a small fraction of premutagenic lesions induced by EO cannot be repaired by the NER system of Drosophila. Linear extrapolation from high to low EO exposure led to an underestimation of the mutation frequency actually observed at low doses. The pattern of EO-induced ring chromosome loss (CL) differed in two respects from that observed for forward mutations: (a) an increase in CL frequencies was observed only at the two highest EO exposure levels, and (b) inactivation of the NER pathway by the mus201 mutant had no measurable effect on the occurrence of CL. The absence of a potentiating effect of mus201 on EO-induced clastogenicity suggests the formation of clastogenic DNA lesions not causing point mutations, and which are not repaired by NER. Consistent with an inversed correlation of reactivities towards N7-guanine and chain length of 1,2-alkyl-epoxides, the relative mutagenic efficiencies of EO:PO:BO are 100:7.2:1.8 for the NER⁺ groups, and 100:20:0.7 in the absence of NER. Although in Drosophila germ cells EO is also more effective as a clastogen than PO, the difference (EO:PO=100:58) is much smaller than for recessive mutations. These results provide another argument that DNA lesions generating base substitutions as opposed to those causing clastogenic damage may not be the same for these agents.     

The effect of cyanide and azide on the mutagenic activity of the pyrrolizidine alkaloid heliotrine inDrosophila melanogaster

Summary Neither cyanide nor azide were found to increase the spontaneous sex-linked lethal frequency or the chromosome breakage frequency inDrosophila. However, both inhibitors produced a slight increase in the heliotrine induced sex-linked lethal and chromosome breakage frequencies in all broods except for the possible exception of the first, the increase for the combined brood totals being statistically significant. Possible mechanisms underlying the enhancing effects of both cyanide and azide are discussed.     

Mutagenicity studies in Drosophila melanogaster with Lannate 20

Lannate 20 a carbamate pesticide was evaluated for its mutagenicity in Drosophila melanogaster by the sex-linked recessive lethals and chromosome II-III translocation tests by continuous larval feeding. The 3 sublethal doses of 0.2, 0.4 and 0.6 microliter of Lannate per 100 ml of the food medium induced a significant (P less than 0.01) increase in the number of sex-linked recessive lethals over the controls. However, no translocations were observed either in the treated or the control series.     

Mutations at six additional loci of Drosophila melanogaster cause alkylation hypermutability

8 mutagen-sensitive strains of Drosophila melanogaster were examined for their effects on alkylation-induced mutagenesis. Using methylnitrosourea as the DNA-damaging agent and the sex-linked recessive lethal test as the monitor of genetic endpoint, 6 of these strains were shown to be hypermutable following exposure to this alkylating agent. Previous studies of 6 other genes have demonstrated that strains exhibiting alkylation hypermutability are completely defective in repair replication following alkylation-induced DNA damage. The present observations suggest that at least 12 loci may be required for excision repair of alkylation DNA damage in this species.     

Dominant-lethal study of technical-grade hexachlorocyclohexane in Swiss mice

Male Swiss mice, 6-8 weeks old, were given a diet containing technical-grade hexachlorocyclohexane (BHC) at 500 ppm continuously for 4, 6 and 8 months. After the completion of the scheduled exposure period, the males were sequentially mated with 2-3 untreated virgin females at weekly intervals for 8 weeks. The females were autopsied at mid-term pregnancy for evaluation of dominant-lethal mutation. The number of dead implants, including deciduomas and dead embryos, showed a significant increase. Similarly, the percentage fertility and live embryos per female showed a decline when compared with the control     

Formation of DNA adducts and induction of mutagenic effects in rats following 4 weeks inhalation exposure to ethylene oxide as a basis for cancer risk assessment

Ethylene oxide (EO) is mutagenic in various in vitro and in vivo test systems and carcinogenic in rodents. EO forms different adducts upon reaction with DNA, N7-(2-hydroxyethyl)guanine (N7-HEG) being the main adduct. The major objectives of this study were: (a) to determine the formation and persistence of N7-HEG adducts in liver DNA of adult male rats exposed to 0, 50, 100 and 200 ppm by inhalation (4 weeks, 5 days/week, 6 h/day) and (b) to assess dose-response relationships for Hprt gene mutations and various types of chromosomal changes in splenic lymphocytes.N7-HEG adducts were measured 5, 21, 35 and 49 days after cessation of exposure. By extrapolation, the mean concentrations of N7-HEG immediately after cessation of exposure ('day 0') to 50, 100 and 200 ppm were calculated as 310, 558 and 1202 adducts/10(8) nucleotides, respectively, while the mean concentration in control rats was 2.6 adducts/10(8) nucleotides. At 49 days, N7-HEG values had returned close to background levels. The mean levels of N-(2-hydroxyethylvaline) adducts in haemoglobin were also determined and amounted 61.7, 114 and 247 nmol/g globin, respectively. Statistically significant linear relationships were found between mean N7-HEG levels ('day 0') and Hprt mutant frequencies at expression times 21/22 and 49/50 days and between mean N7-HEG ('day 0') and sister-chromatid exchanges (SCEs) or high frequency cells (HFC) measured 5 days post-exposure. At day 21 post-exposure, SCEs and HFCs in-part persisted and were significantly correlated with persistent N7-HEG adducts. No statistically significant dose effect relationships were observed for induction of micronuclei, nor for chromosome breaks or translocations.In conclusion, this study indicates that following sub-chronic exposure, EO is only weakly mutagenic in adult rats. Using the data of this study to predict cancer risk in man resulting from low level EO exposures in conjunction with other published data, i.e., those on (a) genotoxic effects of EO in humans and rats, (b) DNA binding of other carcinogens, (c) natural background DNA binding and (d) genotoxic potency of low energy transfer (LET) radiation, it is not expected that long term occupational exposure to airborne concentrations of EO at or below 1 ppm EO produces an unacceptable increased risk in man.     

Gene expression profile in bone marrow and hematopoietic stem cells in mice exposed to inhaled benzene

Acute myeloid leukemia and chronic lymphocytic leukemia are associated with benzene exposure. In mice, benzene induces chromosomal breaks as a primary mode of genotoxicity in the bone marrow (BM). Benzene-induced DNA lesions can lead to changes in hematopoietic stem cells (HSC) that give rise to leukemic clones. To gain insight into the mechanism of benzene-induced leukemia, we investigated the DNA damage repair and response pathways in total bone marrow and bone marrow fractions enriched for HSC from male 129/SvJ mice exposed to benzene by inhalation. Mice exposed to 100 ppm benzene for 6h per day, 5 days per week for 2 week showed significant hematotoxicity and genotoxicity compared to air-exposed control mice. Benzene exposure did not alter the level of apoptosis in BM or the percentage of HSC in BM. RNA isolated from total BM cells and the enriched HSC fractions from benzene-exposed and air-exposed mice was used for microarray analysis and quantitative real-time RT-PCR. Interestingly, mRNA levels of DNA repair genes representing distinct repair pathways were largely unaffected by benzene exposure, whereas altered mRNA expression of various apoptosis, cell cycle, and growth control genes was observed in samples from benzene-exposed mice. Differences in gene expression profiles were observed between total BM and HSC. Notably, p21 mRNA was highly induced in BM but was not altered in HSC following benzene exposure. The gene expression pattern suggests that HSC isolated immediately following a 2 weeks exposure to 100 ppm benzene were not actively proliferating. Understanding the toxicogenomic profile of the specific target cell population involved in the development of benzene-associated diseases may lead to a better understanding of the mechanism of benzene-induced leukemia and may identify important interindividual and tissue susceptibility factors.