(A) GATA1-cre +/+ transgenic males were crossed to females of the ROSA26 reporter strain (ROSA26 +/−), and embryos were recovered at 0.5 dpc (~16-cell morula stage) and stained with X-gal. Cre-mediated activation of the ROSA26 β-galactosidase reporter allele was detected in all cells in embryos in which both alleles are coinherited.