CDK4 and breast cancer: CDK4/6 are inhibited by inhibitors of CDK4/6 (INK4) such as p16INK4A (p16) and p18INK4C (p18). Inactivation of the INK4-CDK4/6-RB pathway (i.e. loss of INK4 or RB and amplification of cyclin D or CDK4/6) is a common event in breast cancers 13, 39.