The dual-function hamster receptor for amphotropic murine leukemia virus (MuLV), 10A1 MuLV, and gibbon ape leukemia virus is a phosphate symporter.

Previously, we showed that the amphotropic receptor homolog in hamster cells functions as a receptor not only for amphotropic murine leukemia viruses and 10A1 murine leukemia virus but also for gibbon ape leukemia virus (C.A. Wilson, K. B. Farrell, and M. V. Eiden, J. Virol. 68:7697-7703, 1994). Here, we demonstrate that this receptor functions as a sodium-dependent Pi transporter and that Na-Pi uptake can be specifically blocked following infection with either amphotropic murine leukemia virus, 10A1 murine leukemia virus, or gibbon ape leukemia virus.     

Mapping the viral sequences conferring leukemogenicity and disease specificity in Moloney and amphotropic murine leukemia viruses.

The Moloney murine leukemia virus (MuLV) is a highly leukemogenic virus. To map the leukemogenic potential of Moloney MuLV, we constructed chimeric viral DNA genomes in vitro between parental cloned infectious viral DNA from Moloney and amphotropic 4070-A MuLVs. Infectious chimeric MuLVs were recovered by microinjection of recombinant DNA into NIH/3T3 cells and tested for their leukemogenic potential by inoculation into NIH/Swiss newborn mice. Parental Moloney MuLV and amphotropic 4070-A MuLV induced thymic and nonthymic leukemia, respectively, when inoculated intrathymically. With chimeric MuLVs, we found that the primary determinant of leukemogenicity of Moloney and amphotropic MuLVs lies within the 1.5-kilobase-pair ClaI-PvuI long terminal repeat (LTR)-containing fragment. The presence of additional Moloney env-pol sequences with the Moloney LTR enhanced the leukemogenic potential of a chimeric MuLV significantly, indicating that these sequences were also involved in tumor development. Since parental viruses induced different forms of leukemia, we could also map the viral sequences conferring this disease specificity. We found that the 1.5-kilobase-pair ClaI-PvuI LTR-containing fragment of Moloney MuLV was necessary and sufficient for a chimeric MuLV to induce thymic leukemia. Similarly, the same LTR-containing fragment of amphotropic MuLV was necessary and sufficient for a chimeric MuLV to induce nonthymic leukemia. Therefore, our results suggest that specific sequences within this short LTR-containing fragment determine two important viral functions: the ability to transform cells in vivo (leukemic transformation) and the selection of a specific population of cells to be transformed (disease specificity).     

Contribution of the gag and pol sequences to the leukemogenicity of Friend murine leukemia virus.

Friend murine leukemia virus (F-MuLV) is a highly leukemogenic replication-competent murine retrovirus. Both the F-MuLV envelope gene and the long terminal repeat (LTR) contribute to its pathogenic phenotype (A. Oliff, K. Signorelli, and L. Collins, J. Virol. 51:788-794, 1984). To determine whether the F-MuLV gag and pol genes also possess sequences that affect leukemogenicity, we generated recombinant viruses between the F-MuLV gag and pol genes and two other murine retroviruses, amphotrophic clone 4070 (Ampho) and Friend mink cell focus-inducing virus (Fr-MCF). The F-MuLV gag and pol genes were molecularly cloned on a 5.8-kilobase-pair DNA fragment. This 5.8-kilobase-pair F-MuLV DNA was joined to the Ampho envelope gene and LTR creating a hybrid viral DNA, F/A E+L. A second hybrid viral DNA, F/Fr ENV, was made by joining the 5.8-kilobase-pair F-MuLV DNA to the Fr-MCF envelope gene plus the F-MuLV LTR. F/A E+L and F/Fr ENV DNAs generated recombinant viruses upon transfection into NIH 3T3 cells. F/A E+L virus (F-MuLV gag and pol, Ampho env and LTR) induced leukemia in 20% of NIH Swiss mice after 6 months. Ampho-infected mice did not develop leukemia. F/Fr ENV virus (F-MuLV gag and pol, Fr-MCV env, F-MuLV LTR) induced leukemia in 46% of mice after 3 months. Recombinant viruses containing the Ampho gag and pol, Fr-MCF env, and F-MuLV LTR caused leukemia in 38% of mice after 6 months. We conclude that the F-MuLV gag and pol genes contain sequences that contribute to the pathogenicity of murine retroviruses. These sequences can convert a nonpathogenic virus into a leukemia-causing virus or increase the pathogenicity of viruses that are already leukemogenic.     

Basis for receptor specificity of nonecotropic murine leukemia virus surface glycoprotein gp70SU.

Murine leukemia viruses (MuLVs) initiate infection of NIH 3T3 cells by binding of the viral envelope (Env) protein to a cell surface receptor. Interference assays have shown that MuLVs can be divided into four groups, each using a distinct receptor: ecotropic, polytropic, amphotropic, and 10A1. In this study, we have attempted to map the determinants within viral Env proteins by constructing chimeric env genes. Chimeras were made in all six pairwise combinations between Moloney MCF (a polytropic MuLV), amphotropic MuLV, and 10A1, using a conserved EcoRI site in the middle of the Env coding region. The receptor specificity of each chimera was determined by using an interference assay. We found that amphotropic receptor specificity of each chimera was determined by using an interference assay. We found that amphotropic receptor specificity seems to map to the N-terminal portion of surface glycoprotein gp70SU. The difference between amphotropic and 10A1 receptor specificity can be attributed to one or more of only six amino acid differences in this region. Nearly all other cases showed evidence of interaction between Env domains in the generation of receptor specificity. Thus, a chimera composed exclusively of MCF and amphotropic sequences was found to exhibit 10A1 receptor specificity. None of the chimeras were able to infect cells by using the MCF receptor; however, two chimeras containing the C-terminal portion of MCF gp70SU could bind to this receptor, while they were able to infect cells via the amphotropic receptor. This result raises the possibility that receptor binding maps to the C-terminal portion of MCF gp70SU but requires MCF N-terminal sequences for a functional interaction with the MCF receptor.     

The N-terminal domain of the murine coronavirus spike glycoprotein determines the CEACAM1 receptor specificity of the virus strain

Using isogenic recombinant murine coronaviruses expressing wild-type murine hepatitis virus strain 4 (MHV-4) or MHV-A59 spike glycoproteins or chimeric MHV-4/MHV-A59 spike glycoproteins, we have demonstrated the biological functionality of the N-terminus of the spike, encompassing the receptor binding domain (RBD). We have used two assays, one an in vitro liposome binding assay and the other a tissue culture replication assay. The liposome binding assay shows that interaction of the receptor with spikes on virions at 37 degrees C causes a conformational change that makes the virions hydrophobic so that they bind to liposomes (B. D. Zelus, J. H. Schickli, D. M. Blau, S. R. Weiss, and K. V. Holmes, J. Virol. 77: 830-840, 2003). Recombinant viruses with spikes containing the RBD of either MHV-A59 or MHV-4 readily associated with liposomes at 37 degrees C in the presence of soluble mCEACAM1(a), except for S(4)R, which expresses the entire wild-type MHV-4 spike and associated only inefficiently with liposomes following incubation with soluble mCEACAM1(a). In contrast, soluble mCEACAM1(b) allowed viruses with the MHV-A59 RBD to associate with liposomes more efficiently than did viruses with the MHV-4 RBD. In the second assay, which requires virus entry and replication, all recombinant viruses replicated efficiently in BHK cells expressing mCEACAM1(a). In BHK cells expressing mCEACAM1(b), only viruses expressing chimeric spikes with the MHV-A59 RBD could replicate, while replication of viruses expressing chimeric spikes with the MHV-4 RBD was undetectable. Despite having the MHV-4 RBD, S(4)R replicated in BHK cells expressing mCEACAM1(b); this is most probably due to spread via CEACAM1 receptor-independent cell-to-cell fusion, an activity displayed only by S(4)R among the recombinant viruses studied here. These data suggest that the RBD domain and the rest of the spike must coevolve to optimize function in viral entry and spread.     

Neurotropic Cas-BR-E murine leukemia virus harbors several determinants of leukemogenicity mapping in different regions of the genome.

The infectious virus derived from the molecularly cloned genome of the neurotropic ecotropic murine Cas-BR-E retrovirus was previously shown to have retained the ability to induce hind-limb paralysis and leukemia when inoculated into susceptible mice (P. Jolicoeur, N. Nicolaiew, L. DesGroseillers, and E. Rassart, J. Virol. 45:1159-1163, 1983). To map the viral sequences encoding the leukemogenic determinant(s) of this virus, we used chimeric viral genomes constructed in vitro between cloned viral DNAs from the leukemogenic Cas-BR-E murine leukemia virus (MuLV) and from the related nonleukemogenic amphotropic 4070-A MuLV. Infectious chimeric MuLVs, recovered from NIH 3T3 cells microinjected with these DNAs, were inoculated into newborn NIH Swiss, SIM.S, and SWR/J mice to test their leukemogenic potential. We found that each chimeric MuLV, harboring either the long terminal repeat, the gag-pol, or the pol-env region of the Cas-BR-E MuLV genome, was leukemogenic, indicating that this virus harbors several determinants of leukemogenicity mapping in different regions of its genome. This result suggests that the amphotropic 4070-A MuLV has multiple regions along its genome which prevent the expression of its leukemogenic phenotype, and it also shows that substitution of only one of these regions for Cas-BR-E MuLV sequences is sufficient to make it leukemogenic.     

Retrovirus packaging cells based on 10A1 murine leukemia virus for production of vectors that use multiple receptors for cell entry.

10A1 murine leukemia virus can enter cells by using either of two different cell surface phosphate transport proteins, the gibbon ape leukemia virus receptor Glvr-1 (Pit-1) or the amphotropic retrovirus receptor Ram-1 (Pit-2). Glvr-1 and Ram-1 are widely expressed in different tissues, but the relative amounts of each are highly variable. We have developed retrovirus packaging cell lines based on 10A1 virus to take advantage of this dual receptor utilization to improve gene transfer rates in somatic cells of animals and humans, in which the relative levels of the two receptors are not always known. Optimization of the Env expression vector allowed the generation of packaging lines that produce helper-free vector titers up to 10(7)/ml. By interference analysis, we found that a 10A1 pseudotype retroviral vector can utilize Ram-1 for efficient entry into mouse, rat, and human cells and can utilize Glvr-1 for entry into mouse and human cells but not for entry into rat cells. The 10A1 pseudotype vector efficiently enters mouse cells by using Glvr-1, while entry into human cells is much less efficient. Thus, the 10A1 pseudotype packaging cells may be advantageous compared with the standard amphotropic packaging cells because vectors produced by the cells can use an additional receptor for cell entry. These packaging cells will also be useful to further explore the complicated pattern of receptor usage conferred by the 10A1 viral surface protein.     

Identification of envelope protein residues required for the expanded host range of 10A1 murine leukemia virus.

The 10A1 murine leukemia virus (MuLV) is a recombinant type C retrovirus isolated from a mouse infected with amphotropic MuLV (A-MuLV). 10A1 and A-MuLV have 91% amino acid identity in their envelope proteins yet display different host ranges. For example, CHO-K1 cells are resistant to A-MuLV but susceptible to infection by 10A1. We have now determined that retroviral vectors bearing altered A-MuLV envelope proteins containing 10A1-derived residues at positions 71 (A71G), 74 (Q74K), and 139 (V139M) transduce CHO-K1 cells at efficiencies similar to those achieved with 10A1 enveloped vectors. A-MuLV enveloped retroviral vectors with these three 10A1 residues were also able to transduce A-MuLV-infected NIH 3T3 cells. This observation is consistent with the ability of vectors bearing this altered A-MuLV envelope protein to recognize the 10A1-specific receptor present on NIH 3T3 cells and supports the possibility that residues at positions 71, 74, and 139 of the 10A1 envelope SU protein account for the expanded host range of 10A1.     

Identification of mouse chromosomes required for murine leukemia virus replication.

The replication patterns of five ecotropic and two amphotropic strains of murine leukemia virus (MuLV) were studied by infecting 41 Chinese hamster x mounse hybrid primary clones segregating mouse (Mus musculus) chromosomes. Ecotropic and amphotropic strains replicated in mouse and some hybrid cells, but not in hamster cells, indicating that replication of exogenous virus requires dominantly expressed mouse cellular genes. The patterns of replication of the five ecotropic strains in hybrid clones were similar; the patterns of replication of the two amphotropic strains were also similar. When compared to each other, however, the replication patterns of ecotropic and amphotropic viruses were dissimilar, indicating that these two classes of MuLV require different mouse chromosomes for replication. Chromosome and isozyme analyses assigned a gene, Rec-1 (replication of ecotropic virus), to mouse chromosome 5 that is necessary and may be sufficient for ecotropic virus replication. Because of preferential retention of mouse chromosomes 15 and 17 in the hybrid clones, however, the possibility that these chromosomes carry genes that are necessary but not sufficient for ecotropic virus replication cannot be excluded. Similarly, the data indicate that mouse chromosome 8 (or possibly 19) carried a gene we have designated Ram-1 (replication of amphotropic virus) which is necessary and may be sufficient for amphotropic virus replication. Because chromosomes 8 and 19 tended to segregate together and two of the three clones excluding 19 have chromosome reaggrangements, we cannot exclude 19 as being independent of amphotropic virus replication. In addition, because of preferential retention, chromosomes 7, 12, 15, 16 and 17 cannot be excluded as being necessary but not sufficient. Hybrid cell genetic studies confirm the assignment of the Fv-1 locus to chromosome 4 previously made by sexual genetics. In addition, our results demonstrate that hybrid cells which have segregated mouse chromosome 4 but have retained 5 become permissive for replication of both N and B tropic strains of MuLV.     

GRB2 interaction with the ecotropic murine leukemia virus receptor, mCAT-1, controls virus entry and is stimulated by virus binding

For retroviruses such as HIV-1 and murine leukemia virus (MLV), active receptor recruitment and trafficking occur during viral entry. However, the underlying mechanisms and cellular factors involved in the process are largely uncharacterized. The viral receptor for ecotropic MLV (eMLV), a classical model for retrovirus infection mechanisms and pathogenesis, is mouse cationic amino acid transporter 1 (mCAT-1). Growth factor receptor-bound protein 2 (GRB2) is an adaptor protein that has been shown to couple cell surface receptors, such as epidermal growth factor receptor (EGFR) and hepatocyte growth factor receptor, to intracellular signaling events. Here we examined if GRB2 could also play a role in controlling infection by retroviruses by affecting receptor function. The GRB2 RNA interference (RNAi)-mediated suppression of endogenous GRB2 resulted in a consistent and significant reduction of virus binding and membrane fusion. The binding between eMLV and cells promoted increased GRB2-mCAT-1 interactions, as detected by immunoprecipitation. Consistently, the increased colocalization of GRB2 and mCAT-1 signals was detected by confocal microscopy. This association was time dependent and paralleled the kinetics of cell-virus membrane fusion. Interestingly, unlike the canonical binding pattern seen for GRB2 and growth factor receptors, GRB2-mCAT-1 binding does not depend on the GRB2-SH2 domain-mediated recognition of tyrosine phosphorylation on the receptor. The inhibition of endogenous GRB2 led to a reduction in surface levels of mCAT-1, which was detected by immunoprecipitation and by a direct binding assay using a recombinant MLV envelope protein receptor binding domain (RBD). Consistent with this observation, the expression of a dominant negative GRB2 mutant (R86K) resulted in the sequestration of mCAT-1 from the cell surface into intracellular vesicles. Taken together, these findings suggest a novel role for GRB2 in ecotropic MLV entry and infection by facilitating mCAT-1 trafficking.     

Influenza virus receptor specificity and cell tropism in mouse and human airway epithelial cells

Recent human infections caused by the highly pathogenic avian influenza virus H5N1 strains emphasize an urgent need for assessment of factors that allow viral transmission, replication, and intra-airway spread. Important determinants for virus infection are epithelial cell receptors identified as glycans terminated by an alpha2,3-linked sialic acid (SA) that preferentially bind avian strains and glycans terminated by an alpha2,6-linked SA that bind human strains. The mouse is often used as a model for study of influenza viruses, including recent avian strains; however, the selectivity for infection of specific respiratory cell populations is not well described, and any relationship between receptors in the mouse and human lungs is incompletely understood. Here, using in vitro human and mouse airway epithelial cell models and in vivo mouse infection, we found that the alpha2,3-linked SA receptor was expressed in ciliated airway and type II alveolar epithelial cells and was targeted for cell-specific infection in both species. The alpha2,6-linked SA receptor was not expressed in the mouse, a factor that may contribute to the inability of some human strains to efficiently infect the mouse lung. In human airway epithelial cells, alpha2,6-linked SA was expressed and functional in both ciliated and goblet cells, providing expanded cellular tropism. Differences in receptor and cell-specific expression in these species suggest that differentiated human airway epithelial cell cultures may be superior for evaluation of some human strains, while the mouse can provide a model for studying avian strains that preferentially bind only the alpha2,3-linked SA receptor.     

Chemical modification of an ecotropic murine leukemia virus results in redirection of its target cell specificity

An ecotropic virus was chemically modified in order to determine whether its target cell specificity could be altered. We hypothesized that chemical coupling of galactose residues to a virus might permit specific infection of hepatocytes mediated by asialoglycoprotein receptors unique to these cells. To test this hypothesis, we took advantage of the fact that: 1) artificial asialoglycoproteins can be created by chemical coupling of lactose to proteins; and 2) viruses that are ecotropic have a narrow species specificity. An ecotropic, rodent-specific, replication-defective murine leukemia virus containing the gene for beta-galactosidase was chemically modified with lactose to contain 5.9 mumol of lactose per mg of viral RNA. Modified and unmodified viruses were incubated for 5 days with HepG2, a human hepatoma line that possesses asialoglycoprotein receptors, and SK Hep1, a human cell line that does not. As expected from the ecotropism, unmodified virus did not produce beta-galactosidase activity in either cell type. Modified virus did not produce beta-galactosidase activity in SK Hep1 cells. However, modified virus did produce beta-galactosidase activity, 71.2 units/mg of cell protein, in the human receptor (+) HepG2 cells. Interestingly, modification of the virus also resulted in decreased enzyme activity in previously susceptible host rodent cells. Competition with modified virus by an excess of an asialoglycoprotein completely prevented development of enzymatic activity in HepG2 cells. Histochemical treatment of cells with 5-bromo-4-chloro-3-indoyl beta-D-galactoside to detect in situ beta-galactosidase activity demonstrated that only HepG2 cells treated with modified virus were positive and that 36% of these cells were stained after 5 days. These data indicate that chemical modification of a virus can result in a redirection of the infectivity of the virus toward hepatocyte-derived cells mediated by the presence of asialoglycoprotein receptors.     

Chromosomal assignment of two endogenous ecotropic murine leukemia virus proviruses of the AKR/J mouse strain.

The AKR/J mouse strain is genetically fixed for three different ecotropic murine leukemia virus genomes, designated Akv-1, Akv-3, and Akv-4 (Emv-11, Emv-13, and Emv-14). With recombinant inbred strains and crosses with linkage-testing stocks, Akv-3 and Akv-4 were placed on the mouse chromosome map. Akv-3, which encodes a replication-defective provirus, maps near the agouti coat color locus, a, on chromosome 2. Akv-4, which is replication competent, maps near the neurological mutant gene locus trembler, Tr, on chromosome 11. Akv-1 and Akv-2 (Emv-12), an ecotropic provirus carried by AKR/N but not AKR/J, have previously been mapped to chromosome 7 and 16, respectively. Thus, the four Akv proviruses mapped to date are on four different chromosomes. Akv-3 is the second ecotropic murine leukemia virus provirus to be mapped near the agouti locus. The results are discussed in relation to possible nonrandomness of viral integration.     

Recombinant mink cell focus-inducing virus and long terminal repeat alterations accompany the increased leukemogenicity of the Mo+PyF101 variant of Moloney murine leukemia virus after intraperitoneal inoculation.

We recently showed that different routes of inoculation affect the leukemogenicity of the Mo+PyF101 variant of Moloney murine leukemia virus (M-MuLV). Intraperitoneal (i.p.) inoculation of neonatal mice with Mo+PyF101 M-MuLV greatly enhanced its leukemogenicity compared with subcutaneous (s.c.) inoculation. We previously also suggested that the leukemogenicity defect of Mo+PyF101 M-MuLV when inoculated s.c. may result from the inability of this virus to form env gene recombinant (mink cell focus-inducing [MCF]) virus. In this study, virus present in end-stage tumors and in preleukemic animals inoculated i.p. by Mo+PyF101 M-MuLV was characterized. In contrast to s.c. inoculation, all tumors from i.p.-inoculated mice contained high levels of recombinant MCF virus. Furthermore, Southern blot analyses demonstrated that the majority of the tumors contained altered Mo+PyF101 M-MuLV long terminal repeats. The U3 regions from several tumors with altered long terminal repeats were cloned by PCR amplification. Sequence analyses indicated that the M-MuLV 75-bp tandem repeat in the enhancer region was triplicated. This amplification was also previously observed in mice infected s.c. with a pseudotypic mixture of Mo+PyF101 M-MuLV and Mo+PyF101 MCF virus. The enhancer triplication was an early event, and it occurred within 2 weeks postinfection. Recombinant MCF viruses were not detected by Southern blot analyses until 4 weeks postinfection. Thus, the M-MuLV enhancer triplication event was initially important for efficient propagation of ecotropic Mo+PyF101 M-MuLV. The increased leukemogenicity following i.p. inoculation could be explained if the triplication enhances Mo+PyF101 M-MuLV replication in the bone marrow and bone marrow infection is required for recombinant MCF virus formation.     

Glycosylation-dependent inactivation of the ecotropic murine leukemia virus receptor.

The ecotropic murine leukemia virus (E-MuLV) receptor expressed on Mus dunni tail fibroblast (MDTF) cells is a receptor for all E-MuLVs with the notable of Moloney murine leukemia virus (Mo-MuLV). Substitution of isoleucine for valine at position 214 in the third extracellular region (the putative E-MuLV binding site) of the MDTF receptor molecule allows this molecule to function as a Mo-MuLV receptor (M.V. Eiden, K. Farrell, J. Warsowe, L. A. Mahan, and C. A. Wilson, J. Virol. 67:4056-4061, 1993). We have now determined that treating MDTF cells with tunicamycin, an inhibitor of N-linked glycosylation, also renders them susceptible to Mo-MuLV infection. Two potential N-linked glycosylation sites are present in the third extracellular regions of both the NIH 3T3 and MDTF ecotropic receptors. The glycosylation site at position 229 of the MDTF receptor cDNA was eliminated by substituting a threonine codon for the asparagine codon. Mo-MuLV-resistant human HOS cells, expressing this form of the receptor, are susceptible to Mo-MuLV infection. Thus, our studies suggest that without a glycan moiety at position 229, the valine residue at 214 is no longer restrictive for Mo-MuLV infection. BHK-21 and CHO K1 hamster cells also express glycosylation-inactivated forms of the ecotropic receptor. Sequence analysis of these receptors together with our analysis of MDTF receptor function suggests that a single asparagine-linked glycosylation site is responsible for glycosylation inactivation of these receptors.     

Functional consequences of natural sequence variation of murine cytomegalovirus m157 for Ly49 receptor specificity and NK cell activation

The Ly49H activating receptor on C57BL/6 (B6) NK cells plays a key role in early resistance to murine cytomegalovirus (MCMV) infection through specific recognition of the MCMV-encoded MHC class I-like molecule m157 expressed on infected cells. The m157 molecule is also recognized by the Ly49I inhibitory receptor from the 129/J mouse strain. The m157 gene is highly sequence variable among MCMV isolates, with many m157 variants unable to bind Ly49H(B6). In this study, we have sought to define if m157 variability leads to a wider spectrum of interactions with other Ly49 molecules and if this modifies host susceptibility to MCMV. We have identified novel m157-Ly49 receptor interactions, involving Ly49C inhibitory receptors from B6, BALB/c, and NZB mice, as well as the Ly49H(NZB) activation receptor. Using an MCMV recombinant virus in which m157(K181) was replaced with m157(G1F), which interacts with both Ly49H(B6) and Ly49C(B6), we show that the m157(G1F)-Ly49C interactions cause no apparent attenuating effect on viral clearance in B6 mice. Hence, when m157 can bind both inhibitory and activation NK cell receptors, the outcome is still activation. Thus, these data indicate that whereas m157 variants predominately interact with inhibitory Ly49 receptors, these interactions do not profoundly interfere with early NK cell responses.     

Chromosomes of the murine leukemia virus indicator cell line XC

A cell line derived from the Rous Sarcoma Virus induced rat tumor XC (Svoboda), which was recently utilized as an indicator for the presence of murine leukemia virus growing in mouse cells, has been examined karyologically. The cells differ considerably from each other as well as from the normal rat karyotype (Rattus norvegicus, 2n=42). The modal chromosome number is 41. All cells bear one or more chromosome markers in common as well as non-rat-like chromosomes, but rat-like chromosomes still preserve the identity of species origin.Supported by Contract No. PH 43-63-13 between the University of California and the National Cancer Institute, National Institutes of Health (Special Virus Cancer Program).