UBCH5b (UBE2D2) is an E2 conjugating ubiquitin protein that functions in the ubiquitination of the tumor-suppressor protein p53. It is a promising drug target in cancer immunotherapy. It utilizes biotin-labeled Ubiquitin and a terbium-labeled antibody recognizing the His-tagged UBCH5b protein to complete the TR-FRET pairing. This FRET-based assay requires no time-consuming washing steps, making it especially suitable for high throughput screening applications.