C9orf72 and amyotrophic lateral sclerosis: Many clinical MNDphenotypes, including classical ALS, progressive muscular atrophy and primarylateral sclerosis, are linked to the C9orf72 gene mutation, butgenerally it is characterized by bulbar-onset, cognitive impairment at a relativelyearly age, and accelerated disease progression.21,33-35 More recent insights revealing that the productsof these identified genes are involved in RNA metabolism and protein homeostasisprovides a further mechanistic link in the pathogenesis of this spectrum.36