MKI67 and retinoblastoma: To determine whether drugs suppressed proliferative tumor cones, thus inhibiting tumor growth, we examined co-localization of the cone-specific transcription factor RXRG [36], which is expressed in tumor cells of origin in both RB1–/–- and MYCN-driven RB subtypes [33, 37–39], and the proliferative marker Ki67 in RB170 and RB654 organoids treated with vehicle or selected drugs at their IC50 for 24 h (Fig. 5 C).