C9orf72 and amyotrophic lateral sclerosis: Third, the most common genetic cause of familial ALS and FTD, a hexanucleotide (GGGGCC) repeat expansion in the C9orf72 gene, is thought to functionally compromise the nuclear transport machinery, as several components involved in protein import, protein export as well as mRNA export are strong genetic modifiers of C9orf72 repeat-associated toxicity31–35.