RB1 and acute lymphoblastic leukemia: Examples in T-ALL are cryptic deletions in 9p21 involving the genes CDKN2A/INK4A/p16, CDKN2B/INK4B/p15, and MTAP/ARF/p14 leading to loss of G1 checkpoint control of the cell cycle or the RB1 locus in 13q14, which also plays a role as tumor suppressor gene in cell cycle regulation [9].