The URE is necessary for the normal generation of BFUe in vivo and BFUe proliferation ex vivo. (A) The frequency of each cell type was measured by FACS in E14.5 fetal livers of wild-type and URE−/− embryos. (B) CFUe colonies were grown in methylcellulose supplemented with Epo and counted at day 3. All other colonies were grown in M3234 supplemented with Epo, IL-3, IL-6, Dex, and SCF and counted after 9 d (n = 3, wild type; n = 2, URE−/−). (C) BFUe from E14.5 fetal livers of wild-type and URE−/− embryos were cultured in “proliferation medium” and counted at the indicated times (n = 3). (D) KSL, CMP, BFUe, and CFUe from E14.5 fetal livers of wild-type and URE−/− embryos; Pu.1 mRNA levels were measured by RT-qPCR (n = 6). Data were analyzed by t test using a false discovery rate of 10%. ns, not statistically significant; * q ≤ 0.05; *** q < 0.001.