KLRD1 and myalgic encephalomeyelitis/chronic fatigue syndrome: Although our previous study did not find significant differences in the number of CD57+ NK cells between ME/CFS cases and healthy controls, we were still interested in evaluating the CD57 marker further, based on the observation that CD57 MFIs (mean fluorescence intensity) yielded significant differences between groups (7), perhaps indicating the presence of other as yet unidentified NK cell subsets in ME/CFS (i.e., lack of CD94/NKG2C receptor expression).