The CTLA4:B7-1 TR-FRET Assay is designed to measure the inhibition of CTLA4 binding to B7-1 in a homogeneous 384 reaction format. This FRET-based assay requires no time-consuming washing steps, making it especially suitable for high throughput screening applications. The assay procedure is straightforward and simple; a sample containing biotinylated CTLA4, B7-1 protein, anti-His Tb donor, dye-labeled acceptor, and an inhibitor is incubated for two hours. Then, the fluorescence intensity is measured using a fluorescence reader.