p16INK4a / CDKN2A: Productsp16INK4a (16 kDa Inhibitor of CDK4-a; also MTS1, CDK41 and CDKN2) is a 16 kDa member of the CDKN2 cyclin-dependent kinase inhibitor family of molecules. It is widely expressed (although not in skeletal muscle) and serves as a negative regulator of cell proliferation. It does so by associating with CDK4 or 6, thereby blocking cyclin binding and subsequent Ser/Thr kinase activity. Human p16INK4a is 156 amino acids (aa) in length. It contains four "L" shaped ankyrin repeats (aa 11-139) that interact with cyclin. There are at least two splice variants for p16INK4a. One is termed p12 and shows a 65 aa substitution for aa 52-156; the other simply shows an alternate start site at Met52. Full-length human p16INK4a shares 63% aa identity with mouse p16INK4a.