Updated project metadata. To characterise the abundance of ubiquitylated Tau peptides in the presence of active or inactive variants (wt 196-565 vs. C221A) of the broad spectrum deubiquitylase USP21. Tau peptides are obtained from Tau fibrils purified from post mortem human brains diagnosed with Alzheimer’s. Their detection is facilitated by previous training of the mass spectrometer with in vitro ubiquitylated Tau. The same samples were used to evaluate the percentage of peptides modified with ubiquitin