Employing clinical data and biological samples for genetic analysis gathered from the Mexican Adolescent Mental Health Survey, we tested the hypothesis that the risk for developing clinical depression would be dependent on the individual and/or cumulative effect of psychosocial adversity factors but moderated by genetic variants; this outcome should be particularly evident for those individuals bearing the BDNF Met allele (i.e., Met/Met and Met/Val) and/or homozygous for the SLC6A4 short allele. The gene discussed is SLC6A4; the disease is major depressive disorder.