Cyclin-dependent kinase regulatory subunit; recognizes TP phosphosites, and functions as a phosphoadaptor that anchors Cdc28p-cyclin complexes onto their substrates, providing specificity and processivity; required for G1/S and G2/M phase transitions; mediates Sic1p phosphorylation and destruction at S phase onset and proteolysis of M-phase targets; regulates transcription by recruiting the proteasome to target gene promoters; human homologs CKS1B and CKS2 each complement the cks1 null mutant