Molecular correlates of S phase entry. ( A ) Cells from five individual subclones expressing inducible MEK1* were serum-starved and restimulated with ZnSO 4 or FBS for 16 hr, and the S phase fraction was determined by using flow cytometry to estimate the DNA content of propidium iodide-stained nuclei. ( B ) Serum-starved cells (subclone 3) stimulated with ZnSO 4 or FBS were assayed at 4-hr intervals for DNA content ( Upper ). Cyclin A, p21 Cip1 , and p27 Kip1 levels were determined by immunoblotting. Precipitates recovered with antibodies to cyclin E and CDK2 were assayed for histone H1 kinase activity.