C9orf72 and amyotrophic lateral sclerosis: Although, DPRs are toxic in both cell culture and animal models, with the arginine containing poly-GR and poly-PR peptides as the most toxic ones (reviewed by Freibaum and Taylor, 20178), the exact pathological mechanisms by which these DPRs contribute to neurodegeneration in C9orf72-ALS/FTD patients remains disputed.