Secondary CFU-S analysis for HaMDR1-transduced primitive hematopoietic cells. At time points from 10 to 24 weeks posttransplant, primary recipients from MDR1 versus DHFR competitive repopulation mice (n = 6) and MDR 15 were killed and BM cells were injected into secondary recipients. Day 12 CFU-S were obtained and DNA was prepared for Southern blot analysis. (A) DNA was restricted with EcoR1 and hybridized with a full-length MDR1 probe that allowed for simultaneous identification of a known fragment size and integration-site analysis. (B) A band of the expected size (3,464 bp, arrow) was seen in all CFU-S examined (88 of 88) from seven individual primary mice. Seventeen representative CFU-S are shown from primary mice nos. 5, 6, and 7 from Fig 5. Negative controls included CFU-S from mice transplanted with untransduced BM and showed two bands presumably from the endogenous MDR genes. Multiple integration patterns could be seen in CFU-S from mice receiving cells from the same expansion pool. Mouse no. 6 in particular showed engraftment with multiple stem cell clones.