CIB1 and neoplasm: By the end of the G2 phase, CDK1 associates with cyclin B. Uncontrolled CDK activation can lead to continuous cell division, contributing to tumor development; this activity is regulated by CDKIs, such as the CIP/KIP family (P21, P27, P57) and the INK family (P15, P16, P18, P19) 41.