Candidate genes of interest implicated in depression and social affect were examined, including the serotonin transporter slc6a4 and receptors for the neuropeptides oxytocin (oxtr) and vasopressin [the main receptor, avpr1a; for reviews, see Beck (2008); Neumann and Landgraf (2012)]. Here, SLC6A4 is linked to major depressive disorder.