In receiver operating curve (ROC) analyses, the best diagnostic accuracies of serum TDP-43 levels were observed between C9-HRE carriers and HC (area under the curve (AUC) = 0.765, with 72% sensitivity and 75% specificity, cutoff 174 pg/mL) and between FTD-MND and HC (AUC = 0.848, with 72% sensitivity and 89% specificity, cutoff 174 pg/mL). Here, C9 is linked to frontotemporal dementia.