Rate of expansion of CRUs from W⁴¹/W⁴¹ fetal liver after their transplantation into irradiated adult recipients. Results shown are the mean (± SEM, data pooled from 4 experiments) number of donor CRUs per recipient, as determined by secondary limiting dilution CRU assays of cells regenerated in irradiated Ly5 congenic primary recipients of 10 CRUs from E14.5 W⁴¹/W⁴¹ fetal livers. For comparison, the results from similarly treated recipients of 10 CRUs from wild-type fetal livers (wild-type FL, dotted line) and +/+ adult bone marrow (ABM, dashed line) as described in Bowie et al³  are also shown.