C9orf72 and frontotemporal dementia: The most common causes of either ALS, FTD, or combined ALS and FTD (ALS-FTD), even within the same families, are pathogenic hexanucleotide repeat expansions in C9ORF72. Nearly 25–40% of all familial ALS and FTD cases carry this mutation and 5–7% of sporadic cases—cases without an established family history of neurodegenerative disease—also screen positive for C9ORF72 pathogenic expansions (DeJesus-Hernandez et al., 2011; Renton et al., 2011).