SCL level determines the output of hematopoietic progenitors. (A) WT bone marrow cells expressing either SCL, ΔbSCL, or control empty vector were plated in suspension culture with SF and IL-11 in selective media (G418). Samples were taken at the indicated times and plated in methylcellulose, and colonies were scored 9 days later. Represented are the cumulative growth curves of multipotent progenitors (CFU-GEMM) (upper panel) or MEPs (BFU-E/Meg) in culture. (B) Bone marrow cells expressing the indicated genes were plated in methylcellulose, and multipotent colonies were aspirated 9 days later, dispersed into single-cell suspensions and counted to determine colony size. Data shown (A-B) are typical of at least 2 independent experiments. (C) Western blot of total protein extracts from unfractionated bone marrow was used to assess SCL protein levels in WT and SCL^tg mice. α-Protein tyrosine phosphatase 1D was used as a loading control. (D-E) Bone marrow cells from heterozygous SCL^tg mice and WT littermates were cultured in methylcellulose in the presence of increasing concentrations of SF (D) or IL-3 (E). Multipotent colonies (CFU-GEMM) were scored 9 days later. Data represent the mean ± SD of 3 independent experiments performed in duplicate or quadruplicate. Data were analyzed using a nonlinear regression curve fitting routine (ALLFIT). Estimates of the half-effective concentrations of the different ligands on progenitors from SCL^tg mice or WT littermates were as follows: 0.7 ± 0.4 (SCL^tg) and 6 ± 3 (WT) ng/mL of SF for CFU-GEMMs or BFU-Es, and 0.2 ± 0.2 (SCL^tg) and 0.2 ± 0.1 (WT) ng/mL of IL-3. *P < .05; **P < .01.