Hexanucleotide (GGGGCC) repeat expansions in C9ORF72 (chromosome 9 open reading frame 72) are the most common cause of frontotemporal dementia and amyotrophic lateral sclerosis (c9FTD/ALS). These disorders are characterized by intracellular inclusions, but the mechanism underlying their formation is unknown. GGGGCC repeats lack ATG start codons, and Mori et al. found that a rare form of repeat-associated non-ATG (RAN) translation of the C9ORF72 hexanucleotide expansion resulted in various dipeptide repeat (DPR) proteins that formed insoluble aggregates. DPR proteins (mostly poly-(Gly-Ala)) were found in inclusions in the hippocampus and cerebellum from patients with ALS carrying the C9ORF72 mutation but not in those who did not. Ash et al. used immunohistochemical analysis to determine the presence of RAN translation products in neuronal inclusions from patients with c9FTD/ALS. Highly selective antibodies were used to detect RAN translation products including poly-(Gly-Ala) in nuclear inclusions in the hippocampus and cerebellum from subjects with c9FTD/ALS but not in those with other disorders. Such antibodies could serve as important biomarkers for FTD/ALS and suggest possible therapeutic targeting of production and aggregation of RAN-translated peptides.