Spontaneous loss of oncogenicity has been found to occur in cultured mouse myeloma cells. A cell line SLU-J27, established from mouse myeloma MOPC-315, was shown to be quite similar in karyotype and tumorigenicity to the tumor of origin. Both contained two markers previously shown to be consistently present in another myeloma, MOPC-21, and in five derived cell lines. These were: a No. 12 chromosome with an extra terminal band and a partially deleted No. 15 chromosome. The SLU-J27 cell line contained two populations that differed in having one or two X chromosomes. When this cell line was inoculated into mice, the resultant tumors were all derivatives of the “2X” population. With time in culture the 2X population gained two new markers and the oncogenicity of the cell line decreased. 5-Bromodeoxyuridineresistant lines derived from SLU-J27 had only the 2X modal karyotype. All centric fusions were between homologs, with one possible exception, suggesting homolog association.

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Supported in part by Grants CA 13595, CA 13625, and HD 03298.

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