C9orf72 and amyotrophic lateral sclerosis: Findings from bacterial artificial chromosome (BAC) transgenic mouse models, which express expanded C9orf72 repeats at lower levels than in our (G4C2)149-mice and (G4C2)66-mice (reviewed [3]), also suggest that repeat RNA and/or DPR protein concentrations influence the development of key c9FTD/ALS features.