SELL and myalgic encephalomeyelitis/chronic fatigue syndrome: Using monozygotic twins to control for genetic differences, Sabath et al. reported that ME/CFS cases and their respective twins displayed a trend of increased circulating CD62L(+) T cells in several T-cell subsets.46 Other studies suggest that polymorphisms observed in subjects with ME/CFS may associate with the sleep abnormalities and the neurological dysfunction associated with this disease.