Thus, autoantibodies against the muscarinic cholinergic receptor, mu-opioid receptor, 5-hydroxytryptamine (5-HT; serotonin) receptor 1A and dopamine receptor D2 have all been detected in ME/CFS patients [218] (for a review see [9]). This evidence concerns the gene OPRM1 and myalgic encephalomeyelitis/chronic fatigue syndrome.