RFC1 and motor neuron disorder: In summary, we present repeat expansions in RFC1 in about 3% of individuals with a clinical main phenotype of MND (i.e., two cases with ALS according to existing Gold Coast Criteria and one patient with PLS, respectively upper motor neuron dominant ALS), emphasizing that a pure or predominant motor neuron disease might be another extreme phenotype of RFC1 spectrum disorder (see Suppl.