A potent, irreversible and specific inhibitor of deubiquitinating enzymes (DUBs) that is prepared by chemical synthesis. It is N-terminally tagged with an HA-tag (YPYDVPDYA), which allows for sensitive identification or purification of DUBs since it is specifically recognized by anti-HA antibodies. The HA tag is separated from the Ub N-terminus by two aminohexanoic acid (Ahx) linkers for efficient recognition of the HA tag.