COMT and psychotic disorder: Estrada et al. (290) showed a dose-effect of COMT polymorphism on the age of onset of psychosis among cannabis users: individuals who were homozygous for the Val allele of COMT had the earliest age of onset of psychotic disorders at 15.4 years; homozygotes for the Met allele had the latest age of onset at 18.8 years; heterozygotes with intermediate enzymatic activity, had an age of onset of 17.1 years.