Normal numbers of hematopoietic progenitors in induced mutant mice. (A) Knock-out and control littermates show similar numbers of CFU-GM and BFU-E colonies. Following induction of the mice (n = 5 per genotype), 30 000 BM cells for CFU-GM and 150 000 cells for BFU-E were plated out per milliliter semisolid medium, and colonies were scored after 7 days in culture. ▪ indicates TβRI null; □, control. (B) TβRI null progenitors and stem cells are resistant to TGF-β–mediated growth inhibition. TGF-β1 (10 ng/mL) was added to some of the cultures, and the number of colonies compared with cultures without exogenous TGF-β was calculated as the percentage of values. ▪ indicates TβRI null; □, control. (C) CFU-S activity of knock-out BM cells is normal. Lethally irradiated mice were injected with 75 000 BM cells from induced knockouts and controls (n = 7 per genotype), and macroscopic spleen colonies were counted after 12 days. Data represent means ± SDs, *P < .02 (B); P > .2 (A,C).