C9orf72 and amyotrophic lateral sclerosis: While not all clinical cohorts show all of these characteristics, the inconsistencies may not only be owing to the different populations under consideration, but also to the groups under comparison, as some reports compare C9orf72 ALS with all non-C9orf72 ALS cases, whereas others compare C9orf72 ALS specifically with familial or sporadic non-C9orf72 ALS.