C/EBPϵ and Rb interact in vivo. (A) COS-1 cells were transfected either with an empty vector or a C/EBPϵ expression vector. Western blot analysis was done to detect Rb and C/EBPϵ expression. A GAPDH antibody was used as control for equal loading. (B) COS-1 cell lysates were immunoprecipitated with an Rb antibody and analyzed by immunoblotting with a C/EBPϵ antibody. (C) NB4 and U937 myeloid cells were treated with ATRA for 0, 2, 4, and 6 days and analyzed by Western blot for Rb and C/EBPϵ expression. The blots were stripped and rehybridized with a GAPDH antibody. (D) NB4 and U937 lysates were immunoprecipitated with a Rb antibody followed by Western analysis with a C/EBPϵ antibody.