CKS proteins, for which the original member, p13suc1, was identified as a suppressor of cdc2 alleles in S. Pombe, have long served as a reagent for the purification of p34cdc2, whereas their biological function has remained elusive. Apparently conflicting data derived from different model systems may indicate a diversity of function for these proteins. Several new observations in yeast and Xenopus egg extracts together with new structural information tends to enhance the hypothesis that CKS proteins function to alter the activity of cdc2 at several important points in the cell cycle. Here we review previous observations and recent data that suggest CKS proteins serve as adaptor proteins that modify the functions of cdc2 throughout the cell cycle.