Recently, ultrasensitive techniques, such as electrochemiluminescence ELISA (ECLIA) [165, 195] and single-molecule array (Simoa) [161], have allowed analysis of promising biomarkers directly in plasma or serum, e.g., neurofilament light chain (NfL), a highly useful biomarker of neurodegeneration, or the protein tau phosphorylated at Thr 217 (pT217-tau) as a specific marker for Alzheimer’s disease (AD) [16, 135, 141]. This evidence concerns the gene MAPT and early-onset autosomal dominant Alzheimer disease.