Case reports of neuroblastoma revealed that some individuals are genetically predisposed and that this genetic predisposition may have other consequences. According to a mutation model, two classes of individuals could acquire neuroblastoma. One (prezygotic) was a rare class that carried a dominant gene imparting high risk of the tumor. The other (postzygotic) comprised all other individuals, each at low risk. The model related tumor incidence to germinal and somatic mutation rates and thereby carried implications for environmental modification of tumorigenesis and demographic variation in incidence. Case reports also revealed associations of neuroblastoma with congenital defects and a susceptibility to second tumors. Analogy with retinoblastoma and Wilms' tumor of the kidney suggested that these associations could result from action of a neuroblastoma gene or from chromosomal aberration. One known dominantly inherited condition, von Recklinghausen's disease, could dispose to neuroblastoma and create some associations. According to the two-mutation model, neuroblastoma may have been a single recessive gene disorder at the level of the cell. The phenomena of aganglionosis, neuroblastoma in situ, maturation of neuroblastoma to ganglioneuroma, and spontaneous regression suggested that such a neuroblastoma gene interfered with normal developmental processes. The specificities of this gene and of those for von Recklinghausen's disease and pheochromocytoma suggested that the functiof a membrane macromolecule.