Cyclin D1 and D2 favor unphosphorylated p27^kip1 isoforms. Western blot shows the expression levels of cyclin D1 in Cal51 and BL40 cells (A, left panel). The specificity of the anti–cyclin D1 antibody, LX014, is shown in panel A, right 2 panels. Human cyclin D1, D2, and D3 were all tagged with a 9E10 epitope, and the expression levels of these cyclins in SF9 cells were detected by the mouse monoclonal antibody 9E10 (A, bottom right). 2D gel analysis shows that both cyclin D1 and cyclin D2 favor unphosphorylated p27^kip1. Although cyclin D1 favors unmodified p27^kip1, isoform 1, cyclin D2 binds to both p27^kip1 isoforms 1 and 2 (B).