Since the interaction between the hypothalamo-pituitary-adrenal axis and the 5-HT system is probably mediated partially by the 5-HT receptors in the hippocampus, this result indicates a hypofunction of the 5-HT system and/or a hypersensitivity of the 5-HT receptors in the brain of CFS patients [24, 25, 26]. The gene discussed is HTR5A; the disease is myalgic encephalomeyelitis/chronic fatigue syndrome.