In such studies, the functions and regulatory roles of neuroendocrine systems, such as the oxytocin (OXT)/OXT receptor (OXTR), estrogen/estrogen receptor 1 (ESR1), arginine vasopressin (AVP)/AVP receptor 1a (AVPR1A), corticotropin-releasing hormone (CRH)/CRH receptor 1 (CRHR1) and the prolactin (PRL)/PRL receptor systems, in maternal behavior and depression have been elucidated [1, 32–36]. The gene discussed is PRL; the disease is major depressive disorder.