This fully functional N-terminal HA-tagged ubiquitin protein allows for the convenient detection or affinity purification of ubiquitinated proteins in vitro. The HA peptide sequence (YPYDVPDYA) is an epitope derived from the influenza hemagglutinin protein. This tag is specifically recognized by anti-HA antibodies and anti-HA-agarose. Ubiquitin is a 76 amino acid, highly conserved nuclear and cytoplasmic protein. It is found exclusively in eukaryotes, becomes covalently attached to substrate proteins by enzymes in the Ubiquitin-Proteosome Pathway (UPP) and has a major role in targeting cellular proteins for the ATP-dependent degradation by the 26S proteosome. Ubiquitination also affects proteosomeindependent events such as protein localization, activity and function.