SF dose-response curves for the in vitro self-renewal of wild-type and W⁴¹ CRUs from FL and adult BM. Data shown for each source of CRUs are expressed as a percent of the number of CRUs in the cultures from which the maximum number of CRUs were recovered. CRUs analyzed were from cultured +/+ fetal liver (FL, open circles), W⁴¹/W⁴¹ fetal liver (W41 FL, solid circles), and W⁴¹/W⁴¹ bone marrow (W41 BM, solid squares) cells. Data for W⁴¹/W⁴¹ was derived from experiments in which the culture medium also contained 20 ng/mL IL-11. Results for wild-type (+/+) bone marrow (dashed line) is redrawn from Audet et al¹³  and reflects conditions that also included varying concentrations of IL-11 and FLT3-ligand in addition to varying concentrations of SF, and cultures were maintained for 10 rather than 2 days.