A possible endomitotic origin of trisomy of chromosome 13: karyological implications for murine mammary tumorigenesis.

The chromosome analysis of mammary tumors of mice by the Giemsa banding procedures yielded the following results: (1) Mammary tumors induced by urethan displayed the same chromosome aberration of trisomy of chromosome 13 as spontaneous mammary tumors; (2) By analyzing tumor cells with translocated chromosomes, genes assumed to be involved in mammary tumor development were mapped in the proximal region of chromosome 13; (3) Polyploid tumor cells manifested heteromorphism among homologous chromosomes which segregated regularly. The analysis of its segregation indicated that trisomy of chromosome 13 was derived from endomitosis and loss of a chromosome. The possible mechanisms of the genesis of the endomitosis were put forth based on a probable role of replicator played by a long terminal repeat of the viral mammary tumor genome integrated into chromosome 13.