Leukemogenicity of clonal isolates of murine leukemia viruses

The leukemogenicity of three types of cloned, in vitro grown murine retroviruses was studied. Two Moloney virus clones caused leukemia, as did five clones of the B-tropic endogenous virus of BALB/c mice. Neither of two clones of N-tropic BALB/c virus caused leukemia in Fv-1n/n mice, and the viruses were not recoverable from the animals. The ability to induce leukemia therefore appeared to reside in the genome of at least certain nondefective murine retroviruses.