MYCN and retinoblastoma: Extending prior observations in MYCN-amplified or MYCN-overexpressing neuroblastoma and retinoblastoma cells (15, 17), we show that intrinsically high MDM2 sustains MYCN expression in MYCN-amplified SCLC cells but has little or no effect on MYC levels in retinoblastoma, neuroblastoma, SCLC, or medulloblastoma cells.