BL-CFC and hematopoietic progenitor content of FLK-1/SCL populations from day 4 EBs. (A) FLK-1^-SCL/lacZ^- (F^-S^-), FLK-1⁺ SCL/lacZ^- (F⁺ S^-), FLK-1⁺ SCL/lacZ⁺ (F⁺ S⁺) and FLK-1^-SCL/lacZ⁺ (F^-S⁺) populations were isolated by cell sorting. Boxes indicate the percentages of different fractions. (B) Total number of blast colonies (BLASTS; ▪) and secondary EBs (2° EBs; ▦) were scored from the unfractionated (PS indicates presort), F^-S^-, F⁺ S^-, F⁺ S⁺, and F^-S⁺ populations. Data are presented as mean number of blast colonies from 3 dishes. Bars, where visible, represent SEM. (C) Expression analyses of isolated fractions. Each lane represents the amplified products from 1000 single cells. Hybridization was carried out with probes designed against the 3′ UTR of the indicated genes. (D) Total number of hematopoietic colonies in each of the indicated fractions. Colonies were categorized as follows: ERY/P (▧), primitive erythroid; MAC (□), macrophages; E/MAC (▨), erythroid-macrophage; and MIX (▦), multilineage. Data are presented as mean number of colonies from 3 dishes. Bars, where visible, represent SEM.