Human Mouse double minute 2 homolog (MDM2) is an E3 ubiquitin-protein ligase that functions as a negative regulator of T-cell activation. It is a potential drug target in cancer immunotherapy. It utilizes biotin-labeled ubiquitin and a terbium-labeled anti-GST antibody 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.