Thus, the fact that the soluble ET-1 protein might be correlated with the variables of the WT circadian rhythm but not with variables of the motor activity circadian rhythm in ME/CFS indicates that ET-1 and skin temperature are both markers of vasoconstriction/vasodilation processes, which may be impaired in ME/CFS and may be related to the intrinsic endothelial structure of the blood vessel [28]. This evidence concerns the gene EDN1 and myalgic encephalomeyelitis/chronic fatigue syndrome.