PARP15 and major depressive disorder: Two other novel variants, associated with reduced likelihood of quitting, also mapped to genes with previously noted markers for psychiatric and substance use disorders: rs2173763 maps to PARP15 on which several locations have been associated with a broad mood disorder phenotype (Major Depression and Bipolar disorder)113; and rs363222 maps to SLC18A2 which is associated with monoamine neurotransmitter transport (dopamine, norepinephrine, serotonin).