RB1 and retinoblastoma: In agreement with this, the topotecan EC50 of RB170 was significantly higher than that of Y79 or RB1-deficient retinoblastoma cells without MYCN amplification (RB654),17 suggesting lower sensitivity of MYCN-amplified retinoblastoma with intact RB1 to topotecan (for Y79, 389.14 ± 16.19 nM vs. 114.5 ± 50.9 nM, P = 0.0167; for RB654, 79.81 ± 5.31 nM, P = 0.0100).