LanthaScreen Ubiquitin Assay Reagents, high-throughput screening products, either monitor the rate of ubiquitination of target proteins or measure the extent of target protein ubiquitination. By incorporating the TR-FRET donor, terbium (Tb), and acceptor, fluorescein, onto ubiquitin itself, these universal assay reagents can be used to rapidly develop screening assays for ubiquitin conjugating enzymes. The reagents offer high (>0.7) Z’-factor values, can detect either mono- or poly-ubiquitination, and allow for real-time kinetic readout of poly-ubiquitination reactions.