Immortalized cell lines are often clonal. (A) Southern blot analysis of the MSCV integrations present in 15 immortalized cell lines. DNA (8 μg) from each immortalized line was digested with HindIII and hybridized with a neo probe isolated from the MSCV vector. Each band represents a separate MSCV integration. (B) Southern blot analysis of 1 representative immortalized immature myeloid parental line (BM-2a) and its 7 single-cell subcloned lines (BM-2a-1-BM-2a-7).