C9orf72 and amyotrophic lateral sclerosis: The familial form of ALS constitutes 10% of all cases of ALS and to-date the causal genes in over 70% of these families have been identified, the most common being C9orf72, a gene of unknown function, Cu/Zn-dependent superoxide dismutase (SOD1), TAR DNA binding protein 43 (TARDBP) and fused in sarcoma (FUS).