C9orf72 and amyotrophic lateral sclerosis: Recently, expansion of a GGGGCC hexanucleotide repeat in the gene C9orf72 has been identified as the most common genetic cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), two diseases that belong to the general class of disorders referred to as c9FTD/ALS [1,2,3].