Thus, patients with a single deletion in mtDNA and manifesting CPEO only, had significantly lower sNFL levels than patients with CPEO + and a single or multiple deletions (due to a mutation in POLG) in mtDNA in muscle (Table 1 and Supplementary Figure 2; 11.3 ± 2 vs. 24.6 ± 5.3; mean ± SD; p < 0.05). This evidence concerns the gene POLG and External ophthalmoplegia.