Zac1 and TGFβII thus join a growing list of tumor suppressor genes with established roles in retinogenesis (for example, Rb, p53, p27Kip1 [33,35,40,44,47-52]), but are the first tumor surveillance molecules shown to control neuronal number through a negative feedback or 'cell sensing' mechanism. This evidence concerns the gene PLAGL1 and neoplasm.