Pc 1 Duarte, a common variant of a human brain-specific protein, is present in one-third of the control population and has a gene frequency of 0.17. There was a significantly increased frequency of mutant heterozygotes and homozygotes among individuals with depressive disease. This is consistent with a model in which Pc 1 Duarte is the major gene in depressive disease, acting in conjunction with an environmental threshold effect.