The activation of naíve T cells requires two signals, the specific T cell receptor recognition of MHC/Antigen on the surface of the antigen-presenting cell (APC), and the binding of B7-1 (CD80) ligand on the APC with the CD28 receptor on the T cell surface. Conversely, binding of CTLA4 to B7-1 on the T-cell surface results in an inhibitory signal and prevents T-cell activation. CTLA4B7-1 interaction is an important drug target for the regulation of the host's response to cancer. The CTLA4 [Biotinylated]:B7-1 Inhibitor Screening Assay Kit is designed for screening and profiling inhibitors of CTLA4:B7-1 signaling. The key to this kit is the high sensitivity of detection of biotin-labeled CTLA4 by streptavidin-HRP. Only a few simple steps on a microtiter plate are required for the assay. First, B7-1 is coated on a 96-well plate. Next, CTLA4[B] is incubated with B7-1 on the plate. Finally, the plate is treated with streptavidin-HRP followed by addition of an HRP substrate to produce chemiluminescence, which can be measured using a chemiluminescence reader.