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. In cervical cancer cells, CDK6 function has been shown to be altered indirectly by the p16 inhibitor. CDK6 is also overexpressed in tumors that exhibit drug resistance, for example glioma malignancies exhibit resistance to chemotherapy using temozolomide (TMZ) when they have a mutation overexpressing CDK6.