Supershift EMSA in the presence of antibody specific for c-Myb or CBP. (A – C). Nuclear extracts (5 μg) from K-562, 3201 or FEA cells were incubated with the radiolabeled GS945 probe (2.4 ng) representing the 21-bp triplication from the FeLV-945 LTR. Shown in each panel is specific complex formation in the presence of nuclear extract (closed circle), and with the addition of monoclonal antibody to c-Myb or CBP (4 μg). Reduced mobility of the complex (supershift) is indicated (asterisk). (D) shows the same reactions performed with nuclear extracts from FEA cells in which c-Myb was exogenously overexpressed.