We found a significantly increased frequency of the PTPN22 rs2476601 risk allele A, that confers susceptibility to autoimmunity, in ME/CFS patients (allele frequency (AF) 12%, odds ratio (OR) 1.50, p = 0.033) compared to healthy controls (AF 8%, n = 201; Table 3). Here, PTPN22 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.