To test whether an MN-restricted ‘double dose’ of mutant FUS accelerates ALS-related pathology, we compared mice expressing either one or two copies of mutant FUS in MNs at 1 year of age—a time point at which MN loss is not yet detected in heterozygous mutant FUS animals (Fig. 1c). Here, FUS is linked to amyotrophic lateral sclerosis.