Clonogenic progenitor cell levels in mice with loss-of-function c-Mpl and/or Kit alleles. Colony-forming cells (CFCs) were determined by agar culture of 25 000 bone marrow cells, 100 000 spleen cells, or 10 000 fetal liver cells from wild-type, c-Mpl^–/–, Kit^Wv/Wv, or c-Mpl^–/–Kit^Wv/Wv mice. Shown are the means ± SDs of CFCs per dish. There were significant decreases in both bone marrow– and spleen-derived CFCs from c-Mpl^–/–Kit^Wv/Wv mice. In contrast, CFC levels were normal in all types of fetal liver cells. α indicates P < .001 versus wild type; β, P < .001 versus c-Mpl^–/–; and γ, P < .001 versus Kit^Wv/Wv.