CFU activity of CD31⁺ Lin^– bone marrow subsets. (A) Beta-galactosidase–expressing day-14 spleen colonies produced by the indicated ROSA26 CD31⁺ Lin^– cell populations. (B) CFU activity in mice that received transplants of these 3 populations of cells: 250 CD31⁺ KSL cells, 5000 CD31⁺ Lin^– c-kit⁺ Sca-1^– cells, and 10 000 CD31⁺ Lin^– c-kit^– cells. The corrected number of CFUs per 10 000 input cells is shown. (Error bars indicate SEM; n = 9 per group; *P < .0005; scale bars, 1 mm; ND, not detected.)