INK4

INK4 is a family of cyclin-dependent kinase inhibitors (CKIs). The members of this family (p16^INK4a, p15^INK4b, p18^INK4c, p19^INK4d) are inhibitors of CDK4 (hence their name INhibitors of CDK4), and of CDK6. The other family of CKIs, CIP/KIP proteins are capable of inhibiting all CDKs. Their function, inhibiting CDK4/6, is to block progression of the cell cycle beyond the G₁ restriction point. In addition, INK4 proteins play roles in cellular senescence, apoptosis and DNA repair.

INK4 proteins are tumor suppressors and loss-of-function mutations lead to carcinogenesis.

INK4 proteins are highly similar in terms of structure and function, with up to 85% amino acid similarity. They contain multiple ankyrin repeats.