IL21 and myalgic encephalomeyelitis/chronic fatigue syndrome: The results also suggest that a combination screening to assess serum levels of EBV/HHV-6A anti-dUTPase Ab, activin A, and IL-21 followed by immunophenotyping studies to elucidate TFH and iNKTFH cell frequencies may be useful not only as biomarker signatures to identify patients with ME/CFS but also to aid in subgrouping or stratifying patients.