CDKN1B and retinoblastoma: Once committed to male development the germ cells enter a period of mitotic arrest that is tightly associated with expression of the G1-S phase regulators p27KIP1 (Cdkn1b; cyclin-dependent kinase inhibitor 1B), p15INK4B (Cdkn2b; cyclin-dependent kinase inhibitor 2B) and p16INK4A (Cdkn2A; cyclin-dependent kinase inhibitor 2A) and the activation of the key G1-S phase check point protein RB (Retinoblastoma) [14].