C9orf72 and amyotrophic lateral sclerosis: The major genetic cause of motor neuron disease (MND)1 (also known as amyotrophic lateral sclerosis (ALS)) and frontotemporal dementia (FTD) is an expansion in the number of GGGGCC hexanucleotide repeats in C9ORF72 from less than 15 in the general population to over 20 (and typically hundreds) (1, , –4).