E3 ubiquitin ligases are enzymes that catalyze the final step of the ubiquitination process and play essential roles in targeted protein degradation. Cereblon (CRBN) is the substrate-recognition component of the CRBN/DDB1/CUL4A/RBX1 E3 ubiquitin ligase complex and is widely used in PROTAC (Proteolysis-Targeting Chimera) drug discovery for targeted protein degradation applications. The TR-FRET Cereblon-4C Binding Assay Kit is designed to quantitatively measure the binding interaction between CRBN and its ligands. The assay includes Tag1-CRBN-4C complex, terbium-labeled anti-Tag1 antibody, and fluorescently labeled thalidomide ligand. Binding of CRBN to thalidomide generates a fluorescence resonance energy transfer (FRET) signal detected by measuring fluorescence emission at 665 nm and 620 nm. Compounds that compete with thalidomide for CRBN binding reduce the HTRF signal, enabling evaluation of CRBN-targeting molecules for drug discovery and PROTAC research applications.