Antibodies against PGC1α (Millipore Ab3243), TFAM (Abcam Ab119684), NEF2L2 (Abcam ab31163), ClpP (Sigma HPA010649), Htra2 (R&D Systems AF1458), mtHsp70 (Thermofisher), Hsp60 (BD Transduction Systems 611562), GPS2 21 (link), p62 (Progen GP62-C), Beclin1 (Millipore Ab15417), Pink1 (Abcam Ab23707), LC3-II (cell signaling 4108) and Parkin (Santa Cruz biotechnologies sc-32282) to assess signaling, were used in combination with MTCOI, SDHA and DAPI to assess changes with COX-deficiency in a single section of muscle. Antibodies that did not produce a sufficient signal-to-noise ratio, or were highly correlated with mitochondrial mass, were removed from further analysis. TFAM, Hsp60, GPS2 and Beclin1 were selected for immunofluorescence on serial sections (n = 4) of patients with multiple mtDNA deletions (n = 3, patients 4, 5 and 7, Supplementary Table 1). IMARIS v.7.7.2 (bitplane) was used to assess average fluorescent intensity of TFAM and Hsp60 relative to MTCOI/SDHA ratio in whole COX-positive and COX-deficient fibers. Subsequently average intensity of the foci was compared to a COX-positive region of the fiber.