Checkpoint Ligand Cell Lines are companion products to the Checkpoint Signaling Assays, which have been engineered to exogenously express a cell surface ligand for a specific checkpoint receptor. When co-cultured with its cognate receptor-expressing cells, the ligand expressed on the Ligand Cell Line will engage with the checkpoint receptor on the Checkpoint Signaling Cell Line. This receptor-ligand engagement leads to receptor activation and downstream signaling events, which are detected by the forcing complementation of the two β-galactosidase enzyme fragments: EA-tagged SH2 domain and PK-tagged checkpoint receptor. The resulting functional enzyme hydrolyzes substrate to generate a chemiluminescent signal.