Representative FACS profiles showing β galactosidase activity in (A) E10.5 to E13.5 embryonic blood cells and (B) E11.5 to E18.5 fetal liver cells from SCL^lacZ/w (solid line) and SCL^w/w (dashed line) embryos. (A) β galactosidase activity in SCL^lacZ/w yolk sac–derived erythroid cells decreased after E11.5. The major peaks represent fluorescence derived from primitive erythroid cells and the minor peaks of high β galactosidase activity seen in the E12.5 and E13.5 samples were in the initial wave of definitive, enucleated erythrocytes (see Fig 6). Results are representative of analyses performed on 2 litters of embryos for each time point. (B) In theSCL^lacZ/w fetal livers, the percentage (mean ± standard deviation [SD]) of cells expressing high levels of β galactosidase activity at each developmental time point are shown. Results are representative of analyses performed on 1 to 3 litters of embryos for each time point.