MFN1 and myalgic encephalomeyelitis/chronic fatigue syndrome: This study examines the effects of an Individualised Activity Program (IAP) on self-reported fatigue, respiratory (VO2sumbax and VO2peak), ANS (low frequency to high frequency ratio of R-R interval (LF/HF-RRI) at rest and during HUTT) and mitochondrial (Mfn1 and Mfn2 levels) functioning in CFS patients and the interaction of these outcomes to provide more insight into the disturbance in underlying mechanisms of the exercise effects.