C9 and amyotrophic lateral sclerosis: Nuclear depletion and cytoplasmic aggregation of the TAR DNA-binding protein 43 (TDP-43) is a pathological hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) patients caused by a hexanucleotide repeat expansion mutation in the C9ORF72 gene (C9-ALS and C9-FTD) [35, 37, 43].