Friend murine leukemia virus (F-MuLV) induces a variety of hematopoietic neoplasms 2 to 12 months after inoculation into newborn mice. These neoplasms are clonal or oligoclonal and contain a small number of F-MuLV insertions in high-molecular-weight DNA. To investigate whether different tumors have proviral insertions in the same region, a provirus-cellular DNA junction fragment from an F-MuLV-induced myelogenous leukemia was cloned in lambda gtWES, and a portion of the flanking cellular DNA sequence was used in blot-hybridization studies of 34 additional F-MuLV-induced neoplasms. Three of these additional neoplasms (one myelogenous leukemia and two lymphomas) were found to have altered copies of the flanking cellular sequence. Restriction enzyme analysis of genomic DNA from these tumors revealed that in each case a proviral copy of F-MuLV had inserted into the same 1.5-kilobase region; all proviruses had the same orientation. Using mouse-Chinese hamster somatic cell hybrids, we mapped this common integration region, designated Fis-1, to mouse chromosome 7. Fis-1 is distinct from three oncogenes on mouse chromosome 7, Ha-ras, fes, and Int-2, based on restriction enzyme analysis and blot hybridization. Therefore, Fis-1 appears to be a novel sequence implicated in both lymphoid and myeloid leukemias induced by F-MuLV.