C9 and amyotrophic lateral sclerosis: Cytoplasmic aggregates of the predominantly nuclear TAR DNA-binding protein 43 (TDP-43) are a pathological hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) cases caused by G4C2 hexanucleotide repeat expansions in C9orf72 (C9-ALS/FTD).