To conclude, the HPG axis is vulnerable to numerous factors that include endocrine inputs (effects of prolactin, leptin and β-endorphin); neural signals (such as occur during suckling); inflammatory molecules (including interleukins and cytokines); stress; weight-loss and energy-deficient states; hypothalamo-pituitary lesions (including disconnection hyperprolactinaemia); and numerous external signals such as opiates, AAS, physical trauma and stress. The gene discussed is PRL; the disease is hyperprolactinemia.