Overexpression of cyclin D1, CDK4, and MEK1* induces cyclin A synthesis and CDK2 activation, but not p27 Kip1 degradation. Cells (subclone 2) ectopically expressing cyclin D1, CDK4, and inducible MEK1* were starved and restimulated with ZnSO 4 or FBS. Cells harvested at intervals thereafter (hr) were assayed for cyclin A and p27 Kip1 expression by immunoblotting (top two panels), for cyclin D1-associated p27 Kip1 by immunoprecipitation with cyclin D1 followed by immunoblotting with anti-p27 Kip1 (middle panel), and for cyclin E- and CDK2-associated histone H1 kinase activity (bottom two panels). More than 85% of p27 Kip1 coprecipitated with cyclin D1.