In the present study, we assessed cathepsin D levels in the plasma and plasma sEVs of a relatively large group of FTLD patients, including sporadic patients, genetic patients (intermediate/pathological C9orf72 expansion carriers, GRN heterozygous mutation carriers), and GRN homozygous mutation carriers affected by NCL, a lysosomal storage disorder. The gene discussed is CTSD; the disease is lysosomal storage disease.