E12.5 compound mutant Runx1 embryos and their FLs. (A-D) Representative whole-mount images of wild-type (wt) and Runx1 heterozygous embryos (wt/rd; the Runx1^rd allele is a Runx1-null allele² ), compared with embryos carrying 1 functional P2-Runx1 allele (P1N/rd) or 1 functional P1-Runx1 (and attenuated P2) allele (P2neo/rd). P1N/rd embryos resemble Runx1 heterozygote embryos, indicating that 1 P2-Runx1 allele is sufficient to rescue embryonic lethality and reduced levels of definitive hematopoiesis. P2neo/rd embryos resemble Runx1 null embryos,^1,2,27  showing hemorrhages in the central nervous system (black arrowhead; 3 bleeding/8 embryos) and severe FL anemia (8/8). Scale bar: 1000 μm.