This appears to be the most robust evidence to date of a biological abnormality in CFS. The findings have been mentioned on several threads I thought but the research itself deserved discussion on its own thread. Biomarkers in Chronic Fatigue Syndrome: Evaluation of Natural Killer Cell Function and Dipeptidyl Peptidase IV/CD26 Bottom line: this very large study (n=176 patients) shows a clear biological difference between CFS patients and controls. Natural Killer Cells of CFS patients, on average, show a very marked reduction in the ability to kill target cells compared with controls: only 12% of target cells were killed by CFS patients NK cells compared with 28% for controls. This confirms earlier, smaller studies by the same authors and has since been independently confirmed by another group in 2011.