In vitro proliferation capacity ofPax5^−/− pre-B I cell clones.

Five randomly chosen Pax5^−/− pre-B I cell clones were grown on stromal cells in the presence of IL-7 followed by intravenous injection into appropriate hosts. About 4 weeks later, thePax5^−/− pre-B I cells were reisolated from the bone marrow of the recipient animals by single-cell FACS, regrown in vitro, and retransplanted. The numbers of in vitro cell divisions were estimated by cell counting during the in vitro expansion phases. The different symbols represent different clones.