Among the fluid markers for the detection and monitoring of the preclinical and clinical stages of NDDs, the most established ones are Aβ42, total tau-protein (t-tau), phosphorylated tau-protein (p-tau181), and neurofilament light (NfL) in AD [55], α-syn in PD [56,57,58,59,60,61] and clusterin [62], creatinine, albumin, transactive response DNA binding protein 43 kDa (TDP-43) in ALS [63,64,65], as well as RNA biomarkers [66]. The gene discussed is MAPT; the disease is Alzheimer disease.