Amyotrophic lateral sclerosis (ALS) is a neurodegenerative condition that forms part of the spectrum of motor neurone disease, which results in progressive weakness and has a poor prognosis. Over the last twenty years, mutations in the SOD1 gene were thought to be the most common cause of the familial form of ALS, accounting for 20% of cases. However, in the last couple of years, hexanucleotide repeat expansions within the C9orf72 gene have been discovered to account for up to 40% of familial ALS, and are now recognised to be the most common known cause. We have recently developed a new assay to analyse C9orf72 expansions, and sought to investigate this in samples that had previously been investigated for SOD1 mutations and found to be negative, over the period of 2003 to 2014. A total of 32.9% (23/70) samples were found to carry pathogenic C9orf72 expansions (>30 repeats), while 2.9% (2/70) were borderline (20–30 repeats) and 50.0% (35/70) were normal (