RB1 and retinoblastoma: Differential miR-24:protein ratios between RB1-/- retinoblastomas, fetal retina and adult retina, and RB1+/+ retinoblastomas, combined with ARF-mediated apoptosis and miR-24-regulated ARF expression in WERI-Rb1 cells nonetheless demonstrate a unique mechanism in RB1-/- retinoblastoma tumors through which proliferation could be maintained by miR-24 suppression of ARF.