Although this group is clinically and genetically well-differentiated compared to the ALS population, the use of a cohort of healthy controls could eliminate potential confounding factors that may influence the interpretation of the results; (ii) the relatively small sample size in some of the subgroups may limit the statistical power to detect significant associations (e.g., the small number of C9orf72 ALS patients in the three repeat expansion groups). The gene discussed is C9orf72; the disease is amyotrophic lateral sclerosis.