C9orf72 and amyotrophic lateral sclerosis: Two most commonly studied genotypes in ALS are the GGGGCC hexanucleotide repeat expansions (HRE) in C9orf72 and SOD1. C9orf72 HRE may lead to a spectrum of clinical manifestations spanning from ALS to frontotemporal dementia (FTD) and clinical manifestations are thought to be closely associated with patterns of phosphorylated 43 kDa TAR DNA-binding protein (pTDP-43) burden.