Intermediate repeat expansion length in C9orf72 may be pathological in amyotrophic lateral sclerosis

An expanded hexanucleotide repeat in C9orf72 causes amyotrophic lateral sclerosis and frontotemporal dementia. All studies to date state that patients have a pathological expansion if they carry 30 or more repeats. We analysed the frequency of C9orf72 repeat expansions in a population based cohort of patients with ALS, and demonstrate that patients with between 20 and 30 repeats are phenotypically similar to patients with an expanded repeat length above 30 repeats. We propose that an intermediate repeat length may be associated with features of the C9orf72 phenotype in ALS patients.