The corepressors SIN3A and ETO2 are excluded from the RUNX1/PU.1 complex but not the RUNX1-ETO/PU.1 complex. (A) Increasing concentrations of PU.1 decrease the amount of ETO2 coimmunoprecipitated with RUNX1. 293T cells were transfected with expression vectors for flag-RUNX1, HA-ETO2, and PU.1 in increasing amounts. In the absence of PU.1, ETO2 co-IPs with RUNX1 (lane 3). As PU.1 amounts are increased, PU.1 preferentially co-IPs with the RUNX1 and ETO2 is progressively excluded (lanes 6, 9, and 12). (B) Increasing concentrations of PU.1 decrease the amount of endogenous SIN3A coimmunoprecipitated with RUNX1. Sin3A co-IPs with RUNX1 in the absence of PU.1 (lane 3). As PU.1 concentrations are increased, PU.1 preferentially co-IPs with the RUNX1 and Sin3A is progressively excluded (lanes 6, 9, and 12). (C) Addition of PU.1 to RUNX1-ETO increases ETO2 recruitment. 293-T cells were transiently transfected with expression vectors for flag-RUNX1-ETO, ETO2, and increasing amounts of PU.1. Increasing amounts of PU.1 increased the amount of ETO2 in the RUNX1-ETO/PU.1 complex (lanes 6, 9, and 12). (D) Addition of PU.1 to RUNX1-ETO increases SIN3A recruitment. 293-T cells were transiently transfected with expression vectors for flag-RUNX1-ETO and increasing amounts of PU.1. Increasing amounts of PU.1 increased the amount of SIN3A in the RUNX1-ETO/PU.1 complex.