RB1 and neoplasm: In a majority of human tumors, the G1/S checkpoint is lost, for example by loss of pRB or the CDK inhibitor p16INK4A, or by overexpression of Cyclin D1 (Ho and Dowdy, 2002; Weinberg, 2007) and insensitivity to antigrowth signals is an hallmark of tumor cells (Hanahan and Weinberg, 2000).