MSCV-Myc tumors display normal karyotypes and are polyclonal. (A) Spectral karyotyping of malignant spleen cells from 5 MSCV-Myc mice did not reveal any clonal abnormalities. A representative karyotype, 40, XX[10], is shown. (B) Analysis of proviral integration by Southern blot hybridization using an MSCV-specific (Psi region) probe. Digestion of spleen genomic DNA (gDNA) with an enzyme (XbaI) that cuts twice in the provirus releases a single 4.2-kb proviral band in MSCV-Myc mice (arrow, lanes 2-8), demonstrating the presence of provirus in affected tissues but not in Balb/c control (lane 1). Digestion of genomic DNA with an enzyme (BglII) that cuts once in the provirus reveals polyclonal smearing and oligoclonal bands. More than 15 individual murine leukemias were analyzed and no clonal tumors were found. A representative blot is shown. Lanes 2, 3, 4, 7, and 8 are from primary MSCV-Myc/Balb/c mice. Lanes 5 and 13 are from primary MSCV-Myc/ Black6 spleens and lanes 6 and 14 are gDNA from an isolated granulocytic sarcoma. Lanes 1 and 9 are Balb/c control splenocytes.