Sequence extending from the start codon in exon 2 to the EcoRI site in exon 3 was replaced with a construct containing human GATA3 cDNA sequence followed by a floxed PGK-neo cassette. Northern blot analyses of cell lines derived from mutant embryos indicated that a transcript comprised of both GATA3 and Gata1 sequence was expressed in erythroid cells (but not in myeloid cells). The presence of a protein translated from GATA3 sequence was verified by immunostaining. No Gata1 protein was detected.