RB1 and neoplasm: Our results suggest that transcriptional regulation by epigenetic modifiers and neural progenitor transcription factors distinguishes transformed RB1 mutant tumor cells from homozygous mutant brain tissue harboring a simple loss-of-function mutation in rb1. We next performed phenotypic mutant analysis in the developing zebrafish larval brain to understand the role of rb1 and the chromatin remodelers rbbp4 and hdac1 in normal neurogenesis.