Inactivation of the conditionally targeted SCL gene during fetal development by Tie2Cre expression. (A-D) FACS analysis of Tie2, SCL, and CD41 expression in SCL/hCD4 ES cell–derived EBs at 3.6 (A) and 4.6 days of differentiation (B-D). Numbers indicate the percentage of cells in the gated area. Human CD4 is expressed from the endogenous SCL locus as a reporter of SCL gene activity. Panel C shows a gated CD41+ subpopulation of day 4.6 cells. (E) Two-primer genomic PCR analysis of SCL^fl/wt Tie2Cre+ and SCL^fl/wt Tie2Cre+ embryonic tissues. EM indicates whole embryo; FL, fetal liver; PB, peripheral blood. (F-G) CD31/PECAM whole-mount staining of vascular endothelial cells of E10.0 SCL^fl/wt Tie2Cre+ and SCL^fl/fl Tie2Cre+ embryos (7× and 3× original magnification, respectively, on a Leica MZ6 stereomicroscope). (H-I) E13.5 SCL^fl/wt Tie2Cre+ and SCL^fl/fl Tie2Cre+ embryos. (J-K) May-Grünwald-Giemsa (MGG) staining of peripheral blood cytospins from E13.5 SCL^fl/wt Tie2Cre+ and SCL^fl/fl Tie2Cre+ embryos (40×). (L-M) FACS analysis of erythropoiesis in SCL^fl/wt Tie2Cre+ and SCL^fl/fl Tie2Cre+ E12.5 fetal livers.