ESAM expression correlates with CFU activity. ESAM−/Lo or ESAMHi cells of the Rag1/GFP− ckitHi Sca1+ fraction of E14.5 fetal liver were sorted and subjected to methylcellulose colony formation assay. Numbers of CFUs (A) and morphology of the colonies derived from the indicated CFUs (B) are shown. The results in panel A are shown as mean plus or minus SD. A black bar in panel B represents 500 mm. The data are from one of 3 independent experiments that gave similar results. Significant difference between the 2 population is indicated (**P < .01).