UAS regulatory sequences drive expression of Ubiquitin containing the amino acid replacements G75A and G76A. The mutant protein can be conjugated by the endogenous ubiquitin C-terminus onto the Lys48 residue, forming free ubiquitin chains, but is unable to conjugate onto substrates including endogenous ubiquitin due to the C-terminal mutations. The protein is tagged with Tag:HA.