RBL2 and retinoblastoma: Unlike the penetration of the germline RB1 mutation in human retinoblastoma, mice cannot develop retinoblastoma with similar disruption of the Rb1 gene, even if pRb is completely inactivated, because pRb-related proteins p107 (RBL1) and p130 (RBL2) compensate for functional loss of pRb [84,85].