The variability in presentation and the interaction between age of onset, symptomatology and the calculation of penetrance in Huntington's disease is used as a framework within which to consider whether the various presentations of the functional psychoses could be different manifestations of the effects of a single gene. The phenomenon of the biotype, i.e., the tendency for family members to resemble each other in symptomatology in a manner which is too variable within families to be explained by either genetic or allelic heterogeneity but which is too stable within families to be explained by random variation, is clearly demonstrable in Huntington's disease. In families with multiple cases of psychosis, the tendency for the diagnosis in the proband to prevail in other affected individuals together with the possible occurrence in yet other individuals of different diagnostic categories at above chance levels may also be an example of the biotype phenomenon. While genetic heterogeneity in the functional psychoses probably also occurs, the possible variation in expression of single genes predicts that the correspondence between different genes and diagnostic subtypes would be limited.