C9orf72 and amyotrophic lateral sclerosis: C9orf72 is a mitochondrial inner membrane-associated protein stabilizing mitochondrial complex I assembly,502 and haploinsufficiency and loss-of-function mutation of C9orf72 lead to decreased activity of neuronal mitochondrial complex I.502,503 Given the discovery of C9orf72 hexanucleotide repeat expansions as a critical cause of ALS,504,505 studies have ever since revealed the multifaceted effects of C9orf72.