C9orf72 and amyotrophic lateral sclerosis: Best threshold Cox analysis revealed a significant survival association with C9orf72, which was strongest at 24 repeat units (HR = 1.52, 95% CI = 1.34–1.71, PFDR = 6.65 × 10−11), though a significant association with ALS survival was already observed at repeat lengths above 13 (Fig. 3B, Fig. S18).