The IL2 variant leads to increased secretion of the cytokine and hence proinflammation and has been associated with increased postoperative pain and morphine use [18] and the BDNF variant causes reduced activity-dependent BDNF secretion [33, 34] and is associated with increased depression symptoms in cancer patients [35–37], the opposite to that found here. Here, BDNF is linked to major depressive disorder.