C9orf72 and amyotrophic lateral sclerosis: Further studies are needed to clarify if genotype-specific differences exist independent of disease aggressiveness, as, for example, C9orf72 expansion carriers are known to have a worse prognosis relative to patients with sporadic ALS or other familial mutations (Miltenberger-Miltenyi et al., 2019; Rooney et al., 2019).