Plasma tau truncation profile after chemical extraction and IP in the discovery cohort. Left: Plasma tau peptides concentration profiles obtained from the 36 individuals. Each line corresponds to the peptide profile from one participant. Right: Dots represents averaged tau peptides normalized concentration obtained from the overall cohort. Normalized concentration for each peptide is relative to the sum of the peptides concentrations measured in each participant. Bars represent SD. Label a indicates a decrease of 2N and 1N+2N peptide abundance consistent with 5/5/1 0N/1N/2N contribution in plasma tau. Label b indicates a decrease consistent with the presence of around 10% of phosphorylation on position 181. Phosphorylation on T181 induces a trypsin missed cleavage between residues 180 and –181. This contributes to a decrease of 175–180 and 181–190 peptides abundance proportional to the extent of phosphorylation on T181. Label c indicates a decrease consistent with tau truncation between residues 221 and 226. Cicognola et al. (2019) have reported CSF tau main cleavage occurring at residue 224. Label d indicates a decrease consistent with progressive C terminus degradation of plasma tau from residue 224 to microtubule binding region upstream region.