C9orf72 and amyotrophic lateral sclerosis: Regarding the presence of muscle dysfunction in relation to the expression of other mutant genes linked to ALS, it must be recalled that TDP-43 immuno-reactivity is detectable only in muscle fiber nuclei without any sarcoplasmic TDP-43 aggregation [30,31]; very recently, it has been demonstrated that skeletal muscle contributes to the ALS phenotype also in C9orf72 related cases [32].