PU.1 knock-out fetal liver HSCs are arrested at the transition to the common myeloid and lymphoid progenitor stages. Multicolor FACS analysis was performed in E14.5 fetal livers from PU.1^+/+, PU.1^–/+, and PU.1^–/– embryos. (A) The top panels demonstrate the Sca-1/c-Kit profile of Lin^– cells. PU.1^–/– fetal liver has a decreased number of Lin^–Sca-1⁺c-Kit⁺CD34⁺ HSCs. PU.1^–/– fetal liver lacks CMPs and GMPs as well as their progeny, mature monocytic (CD11b⁺/Gr-1^lo) and granulocytic (CD11b⁺/Gr-1^hi) populations. (B) PU.1^+/+, PU.1^–/+, and PU.1^–/– fetal livers have almost equal numbers of Lin^–IL-7Rα⁺ cells. PU.1^–/– fetal liver lacks Lin^–IL-7Rα⁺Sca-1^loc-Kit^lo CLPs and CD19⁺ early B cells. In both analyses, PU.1^–/+ fetal liver has intermediate numbers of myeloid and lymphoid progenitors. Numbers in each panel represent percentages of the gated population in whole fetal liver cells. FSC indicates forward scatter; Gra, granulocytes; Mo, monocytes; and Lin, lineage.