The transcriptionally active form of Runx1 is required for hematopoietic rescue. P-Sp cells from Notch1-null embryos at E 9.5 were infected with retroviruses containing Runx1 mutants and cultured on OP9 cells for 12 days. Structures of Runx1 mutants are depicted (left column). Runt indicates the Runt domain; m, a binding region for mSin3A; AD, transactivation domain; ID, inhibitory domain; and VWRPY, VWRPY motif. Microscopic representations (center column; original magnification, × 100) and flow cytometric analyses (right 2 columns) of cells produced in the culture. Percentages of cells gated (center columns) and cells in each quadrant (right columns) are indicated.