C9orf72 and Mental deterioration: Some other studies have used estimated years to symptom onset based on mean family age at onset, but a recent article demonstrated that the correlations between age at symptom onset and mean family age at symptom onset were weak for C9orf72 and GRN pathogenic variant carriers, indicating that this might not be a reliable proxy.28 By stratifying according to CDR plus NACC FTLD, we have provided insight into cognitive decline during different disease stages.