Mutations in multiple genes have been found to be linked to ALS, including those that encode the oxidative-stress-related enzyme SOD1, RNA/DNA-binding proteins such as FUS and TDP-43, other proteins, including VCP, UBQLN2, and profilin 1, and the orphan gene C9orf72, the most common cause of fALS. The proteins SOD1, FUS, TDP-43, and some fragments (peptides) derived from C9orf72 have been found to form abnormal, toxic aggregates in patients with ALS. However, whether these aggregates cause the disease is still an open question.