BFU-Es and CFU-Es are enhanced in STAT1^–/– splenocytes. In vitro hematopoietic colony formation was examined in methylcellulose supplemented with 3% kit-ligand conditioned medium and varying concentrations of EPO using splenic cells from wild-type and STAT1^–/– mice at 8 to 12 weeks of age. (A) BFU-Es were enumerated 7 days after plating. (B) CFU-Es were counted 2 days after initiation of the experiment. The data shown are the mean ± SE for 3 animals (*P < .05). EPO concentration/response curve of the growth of (C) BFU-E–derived and (D) CFU-E–derived colonies in cultures of splenocytes from wild-type (▪) and STAT1^–/– (□) mice. The results are presented as a percentage of the maximal colony growth and are the mean ± SE for 3 animals (*P < .05).