Our main results were as follows: (i) Significant poly‐GP levels are detectable in the CSF of 93.3% of the C9orf72 ALS and bvFTD cases but not in 91.6% of the control cases; (ii) rapid poly‐GP immunoassay is useful to detect individuals with a C9orf72 expansion misdiagnosed with other diseases (e.g. AD); and (iii) poly‐GP levels are already increased in asymptomatic stages of the disease, suggesting DPRs may be most important for the early pathogenic events in C9orf72 ALS/FTD rather than driving acute neurodegeneration in late‐stage patients. Here, C9orf72 is linked to Alzheimer disease.