C9orf72 and amyotrophic lateral sclerosis: After adjustment for sex, site of onset and ALS family history, rs4970944 is significantly associated with age of onset in both C9orf72-carriers (P = 0.025, B = 1.6, SE = 0.7; n = 333) and non-carriers (P = 0.015; B = 0.8, SE = 0.3; n = 3697) (Additional file 1: Fig. S6b-6c).