CDKN2A and neoplasm: Single deletion of Cdkn2b (codes for p15Ink4b) in mice does not result in any significant tumor susceptibility [135], while Cdkn2a (codes for p16Ink4a and p19ARF) global knockout animals reach adulthood without any major developmental defects, but begin to spontaneously form tumors at 20 weeks of age [153].