We have therefore typed polymorphisms at the serotonin transporter, dopamine receptor, tryptophan hydroxylase, tyrosine hydoxylase, and monoamine oxidase A (MAOA) loci in 139 unipolar and 131 bipolar patients and investigated associations with gender, number of episodes, age of onset, history of psychotic symptoms, history of suicidal behavior, and history of substance abuse.
A significant correlation between the presence/absence of the disorder and the length of the MAOCA-1 repeat was found in males, but not females, with "long" alleles (repeat length above 115 bp) associated with both increased risk for the disorder and lower age of onset of substance abuse.