Gene-environment interactions (GEI) were recently found to contribute to the etiology of mood disorders and suicidal behavior in children adolescents and young adults. Childhood adversity may produce a biological and clinical diathesis for mood disorder and suicidality that endures into adulthood. Life events predict depression and suicidal ideation or a suicide attempt in children, adolescent and young adult carriers of the S allele of the 5-HTTLPR polymorphism. This finding was replicated in children, adolescents and young adults. We reported on a third functional allele in the serotonin transporter gene (A trialleleic polymorphism) that may give a new clue to the GEI in depression and suicide. Some new biological data supporting the validity of this interaction will be presented.