Dominant binding of β/SMMHC to RUNX1 over PEBP2β COS7 cells were transfected with expression plasmids for either RUNX1, β165, or β/SMMHC, and WCE was prepared from each transfected culture as a protein source. One quarter of the amounts of RUNX1, β165, and β/SMMHC used in lanes 4 to 21 is shown in lanes 1, 2, and 3, respectively. Specificity of the antibodies is shown in lanes 4 to 9. A fixed amount of WCE containing RUNX1 (lanes 10-21) was mixed with increasing amounts (2.5 ×, 5 ×, and 25 ×) of β165 (lanes 10-12 and 16-18) or β/SMMHC (lanes 13-15 and 19-21) containing WCE and was subjected to IP/Western blotting. IP-α indicates immunoprecipitation with anti-RD; IP-β, immunoprecipitation with anti-Cbfβ (β141.4.1).