OPRM1 and myalgic encephalomeyelitis/chronic fatigue syndrome: Two of the three CpG sites showed significantly higher methylation in the CFS/FM group compared to the healthy controls (Table 2), which translated to a significant difference in mean OPRM1 methylation (univariate test for group: F = 5.136; mean difference = 0.524 [95% C.I. 0.065–0.983]; p = 0.026).