C9orf72 and amyotrophic lateral sclerosis: In the initial discovery series, TIA1 mutations were identified in 2.2% of familial ALS and 0.4% of sporadic ALS cases [19]; far less frequent than the most common genetic cause of ALS, which is the C9orf72 repeat expansion (responsible for 37% of familial and 6% sporadic ALS [26]), but similar to the frequencies reported for VCP and SQSTM1 mutations and more common than many other genetic variants that have been associated with ALS [2].