CDKN2A and breast cancer: The cyclin D-CDK4/6-INK4-Rb pathway is frequently deregulated in breast cancer via CCND1 (cyclin D1) amplification (29–58%), CDK4 (14–25%) and CDK6 amplification [18], p16 loss (49%) [19], and TP53 inactivation (12–84%) [18].