RB1 and retinoblastoma: The ratio of ARF protein to mRNA levels (normalized relative to FR) revealed that some RB1-/- retinoblastoma cell lines (RB 247, RB 381, RB 1021, WERI-Rb1) demonstrate much lower ratios of protein:mRNA than both human fetal and adult retinas, RB1+/+ cell lines (RB 3823 and 522), and cancer cell lines HeLa and SKOV3 (Figure 1C), indicating a defect in ARF protein translation.