Mammalian G1 phase progression is regulated by G1 cyclin and cyclin-dependent kinases (cdks). Cdk4 or cdk6 is associated with D-type cyclins while cdk2 binds to cyclin E or cyclin A to become an independent and essential kinase [1,2]. Cdk3 is another putative G1 cdk, whose cyclin partners have not been identified [3]. In vitro, cdk3 is an active kinase in association with either cyclin E or cyclin A [4,5]. In eukaryotes, overexpression of a dominant-negative cdk3 induces G1 arrest, which is not rescued by upregulation of wild-type cdk2, suggesting that the function of cdk3 is distinct from that of cdk2.
