WBB6F1-W/W^v spleen contains few BMP4^R cells. Spleen cells from WBB6F1 control, WBB6F1-W/W^v, C57BL/6 control, and C57BL/6-W^v/⁺ mice were plated in media containing Epo (3 U/mL) or Epo + BMP4 (15 ng/mL). The values indicated are the average of 2 mice each done in triplicate. The fold increase in stress BFU-Es when BMP4 was included in the media was calculated by dividing the number of stress BFU-E colonies observed in Epo + BMP4 by the number of stress BFU-E colonies observed in media containing only Epo. Error bars indicate SD.