Inhibition of GATA-1–mediated transcriptional activation by RUNX1-ETO. (A) RUNX1-ETO inhibition of GATA-1 function in transactivation of the αIIb-598 promoter. Cotransfections were performed exactly as in Figure 4A, except that RUNX1-ETO was expressed instead of RUNX1. Results represent the mean ± SEM of 3 independent experiments. Whole cell lysates from duplicate transfections were immunoblotted with the indicated antibodies. (B) Dosage dependency of RUNX1-ETO mediated inhibition of GATA-1. A constant amount of pEF-GATA-1 (1.5 μg) was cotransfected with the indicated amounts of pCMV-RUNX1-ETO. Activation of the αIIb-598-Luc reporter was determined as in Figure 4A. Results represent the mean ± SEM of 3 independent experiments.