C9orf72 and amyotrophic lateral sclerosis: A GGGGCC hexanucleotide repeat expansion (HRE) within the non-coding region of the C9orf72 (C9orf72-SMCR8 complex subunit) gene is the most common known cause of familial and sporadic cases of amyotrophic lateral sclerosis (ALS), a devastating neurodegenerative disorder characterized by the loss of cortical and spinal motor neurons [1–4].