A primitive progenitor is responsible for the long-term expansion of CM-expressing cells. (A) Long-term CD34⁺ CM cells (week 9-15 in vitro) were cultured on the MS-5 stroma line under B cell-differentiating conditions. After 4 weeks, cells were collected and analyzed for GFP expression (indicating expression of CBFβ-SMMHC) and for expression of the B-cell markers CD10, CD19, and CD20. (B) Ten million cells from a week 12 or week 13 CM culture were injected intravenously into NOD/SCI-B₂M^-/- mice. Eight or 9 weeks later, mice were killed and bone marrow from the 4 long bones of the leg was obtained and analyzed for GFP expression and for expression of the human blood cell molecule CD45.