Evidence is mounting that genetic factors may be included in the many determinants of suicide. Clinical studies of psychiatric patients have suggested that risk of suicidal behavior is increased by the presence of family history of suicidality, a claim that is also supported by findings of twin and adoption studies. In addition, molecular genetic studies have reported polymorphisms in the tryptophan hydroxylase gene that is involved in the synthesis of serotonin. The genetic susceptibility to suicide, however, tends to affect individuals only in association with stress or psychiatric illness.