Drug-resistant human PBPCs can engraft NOD/Scid mice (A). The human SEX gene was shown by PCR in the BM of NOD/Scid mice engrafted with as few as 180 to 270 PBPCs enriched by immunoaffinity removal of lineage-positive cells and still viable after exposure to both mafosfamide and taxol (mice 22-25). Higher levels of engraftment were found in the BM of NOD/Scid mouse transplanted with 185 × 10³ PBPCs enriched by immunoaffinity removal of lineage-positive cells and still viable after exposure to both mafosfamide and taxol (mouse 19) and in the BM of mice transplanted with 4 to 233 × 10³ PBPCs enriched by immunoaffinity removal of lineage-positive cells and still viable after 7 days culture in the presence of 5-FU, SCF, and IL-3 (mice 7 and 11). Human SEX gene expression from cells other than human drug-resistant hematopoietic progenitors was excluded, because untransplanted mice and mice transplanted with irradiated L87/4 cells alone were negative. (B) Human SEX gene expression in CFU and CAFC cultures from the BM of mice transplanted with 0.18 to 185 × 10³ PBPCs enriched by immunoaffinity removal of lineage-positive cells and still viable after exposure to both mafosfamide and taxol (mice 19 and 25).