CDKN1B and cancer: It is notable that AAV2 triggered S-phase entry in MCF-7 and MDA-MB-231 cells occurred in the presence of decreased levels of CDK inhibitors p21WAF1, p27KIP1 and p16INK4, conditions which would be expected to enhance entry into S phase, and which could be a mechanism whereby AAV2 competes with the cancer cells for cellular factors for its own transcription.