The cyclin-dependent kinases form complexes with their cyclin partners and with CDK inhibitors. CDK6 and CDK4 associate with the D-type cyclins and target the retinoblastoma protein, allowing passage through the G1/S phase restriction point. CDK6/cyclin D complexes are sequestered in their inactive form through binding to one of the INK4 CDK inhibitor family members. Unlike the INK4 family of cdk inhibitors, the CDK inhibitor p21 Waf1/Cip1 may enhance the association of CDK4 and CDK6 with their cyclin D partners. Synonyms: MGC59692, PLSTIRE