RB1 and retinoblastoma: In contrast to these findings, in animals with inactivation of Rb on a p53-/- background, (NesCre1 Rb lox/lox p53-/- mice) retinoblastomas did not result [69], suggesting that p53 loss can impact upon retinoblastomas that have already been initiated, but that inactivation of a second pocket protein in addition to Rb is still critical.