Inability of C/EBPα to induce myeloid maturation of PU.1^−/− cells. (A) Western blots showing expression of C/EBPα-ER or PU.1-ER(T) in 503 PU.1^−/− cells. Lane 1 in each blot contains extracts from the parental cell line. The filters were probed with human (left) or murine (right) ERα antisera. (B) C/EBPα-ER–expressing cells exposed to E2 and PU.1-ER(T) cells exposed to 4HT for 2 days were subjected to FACS analysis. Percentage monocytes is indicated (% M). (C) C/EBPα-ER–expressing cells exposed to E2 and PU.1-ER(T) cells exposed to 4HT for 3 days were cytospun and visualized by Wright-Giemsa staining. See “Cell culture and transfection” for image acquisition information.