OPTN and amyotrophic lateral sclerosis: However, other rarer FTLD–TDP subtypes (e.g., type D associated with VCP mutations and type E) and other genetically based ALS/FTLD–TDP cases (e.g., associated with mutations in TARDBP, UBQLN2, OPTN, and TBK1) exist and it will be interesting to learn in the future how they will be classified based on pTDP-43S375 immunoreactivity [12, 18].