As for C9orf72, the most prevalent mutation in ALS, characterized by the expansion of the GGGGCC (G4C2) hexanucleotide repeat in the first intron of the C9ORF72 gene [72], observations have pointed to reduced activities of mitochondrial complexes I and V within the frontal cortex of 6-month-old C9orf72-ALS/FTD mice [36]. Here, C9orf72 is linked to frontotemporal dementia.