Retroviral expression of Runx1 rescues hematopoietic defect of Notch1-null P-Sp region. (A) P-Sp cells from wild-type (Notch1^+/+) and Notch1-null (Notch1^–/–) embryos at E9.5 were cultured for 5 days on OP9 cells. (B) P-Sp cells from Notch1-null embryos at E9.5 were infected with mock retrovirus or retrovirus containing Runx1, SCL, or GATA2, and cultured for 12 days on OP9 cells. Microscopic representation (left column; original magnification, × 100). Only cocultured OP9 cells are shown if hematopoietic cells are not produced. Flow cytometric analyses (center and right columns) of cells generated in the culture. Percentages of cells gated (center columns) and cells in each quadrant (right columns) are indicated.