CDK4 and pituitary tumor: The CKIs, p16, p15, p18 and p19, inhibit the activity of the CDKs CDK4 and CDK6, thereby preventing phopsphorylation of Rb, and G1 to S phase transition (Cicenas et al., 2014), and a mouse with a Cdk4 point mutation (Arg24Cys) that resulted in insensitivity to the CKIs (Sotillo et al., 2001), developed tumours of the pituitary, pancreas and testes (Sotillo et al., 2001).