P-Sp of E9.5 CBP homozygous mutant embryos and wild-type littermates were cultured on OP9 cells for 10 days. Nonadherent cells were then assayed for hematopoietic colonies in a methylcellulose medium. The number of hematopoietic colonies was scored after 7 days of culture. In vitro hematopoietic activity of P-Sp culture of homozygous embryos was partially rescued by 100 ng/mL VEGF (+VEGF). Total numbers of colonies (>40 cells) per one explant were obtained from three independent experiments and expressed as the mean ± SEM (n = 3).