SLC6A4 and myalgic encephalomeyelitis/chronic fatigue syndrome: A gene polymorphism analysis in CFS patients by our group demonstrated that the frequency of longer (L or XL) allelic variants of the 5-HT transporter (5-HTT) promoter region was significantly increased compared to that in controls, pointing to elevated 5-HTT expression and low levels of extracellular 5-HT concentrations in CFS patients (25).