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Amplification of endogenous proviral MuLV sequences in radiationinduced osteosarcomas

The endogenous ecotropic provirus of BALB/c mice was found to be amplified in 17 out of 29 radiation‐induced osteosarcomas. In contrast, 19 clonal cell lines established from bone‐marrow cells of a tumor‐bearing mouse, which were used as controls, did not reveal newly acquired ecotropic proviruses. Ecotropic viral RNA was expressed in tumors that showed reintegrated proviruses. DNA probes from 2 tumors, derived from cellular sequences flanking the newly integrated proviruses, did not detect DNA rearrangements in any of the other tumors. The possible role of activated endogenous retroviruses in the development of radiation‐induced osteosarcomas is discussed.