CDKN2A and cancer: While Cdkn3 is expressed mostly in cancer (Nalepa et al, 2013), other CDKIs are categorized into the Cip/Kip family (p21 encoded by cdkn1a, p27/cdkn1b, and p57/cdkn1c) and INK4 family (p16/cdkn2a, p15/cdkn2b, p18/cdkn2c, and p19/cdkn2d) (Besson et al, 2008).