Flow cytometric analysis of yolk sac colonies cultured for 13 days in methylcellulose media containing IL-3, IL-6, and stem cell factor (bold solid line +/+, thin solid line +/−, dotted line −/−). Expression of CD11b (granulocytes, macrophages) is seen in Mll +/+ and +/− colonies, while high-level expression of c-kit (hematopoietic progenitor cells) is seen in Mll −/− cultures. Identical results were obtained in two experiments.