Comparison of β-globin and GATA-1 expression levels in wild-type versus GATA-2^−/− definitive erythroid cells. Levels of β-globin and GATA-1 transcripts in definitive erythroid cells of wild-type colonies (1 and 2) and GATA-2^−/− colonies (3 and 4) were compared by RT-PCR analysis with 2 different PCR cycles. Lanes 1 and 3, 24 PCR cycles for β-globin and 32 PCR cycles for GATA-1; lanes 2 and 4, 26 PCR cycles for β-globin and 34 PCR cycles for GATA-1. Actin and HPRT were used as internal controls for β-globin and GATA-1 expression, respectively.