Fig. 1. (A) Wild-type and H175R M-cyclin associate with cdk2 but not with cdk4 or cdk6 in vivo, whilst K-cyclin associates with cdk2, cdk4 and cdk6. (B) The left panel shows wild-type and H175R M-cyclin complexes with cdk2 immunoprecipitated through the cyclin component and assayed against pRb. In vitro kinase assays were performed in the presence of increasing amounts of p27^Kip and show the reduced sensitivity of wild-type and mutant M-cyclin towards this inhibitor compared with cyclin A. The lower panel shows a bar graph of the decrease in kinase activity due to increased inhibitor concentrations. ³²P counts incorporated into pRb were quantitated on a phosphoimager and normalized to the amount of total kinase activity present when no inhibitor was used.