ES cells with GATA-1.05/Y genotype differentiate into blastlike cells. Differentiation of GATA-1.05/Y ES cells were examined using the OP9/ES cell in vitro differentiation system. (A) Numbers of floating cells produced by GATA-1.05mutant ES clones at both day 8 and 14. (B) Quantitative RT-PCR analysis of GATA-1 mRNA expression in the wild-type and GATA-1.05ES cells. (C and D) Wright-Giemsa staining of the wild-type and mutant day 14 floating cells.