Dimerization of bcl-2 and bax. Lysates from control cells and cells treated with ATRA or taxol were immunoprecipitated with anti–bcl-2 (A) or with anti-bax (B). The immunoprecipitates were separated by 10% to 20% SDS-PAGE gel electrophoresis, transferred to nitrocellulose, and stained with bcl-2. It is evident that both normal and phosphorylated bcl-2 formed heterodimers with bax.