Single cell cultures of TβRI null LSK cells are resistant to TGF-β–mediated growth inhibition and exhibit increased proliferation recruitment under low stimulatory conditions. (A) LSK cells from induced mice (n = 5 per genotype) were seeded in Terasaki plates in serum-free medium supplemented witha cocktail of cytokines (see “Materials and methods”) with or without TGF-β1 (10ng/mL). The numbers of proliferating clones per 120 wells counted at day 12 are shown. ▪ indicates TβRI null; □, control. (B) When cultured in SCF alone, TβRI null LSK cells have a 2-fold increased proliferation recruitment compared with controls. Data represent means ± SEM. *P ≤ .01.