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Interactions between protein ligands and receptors play crucial roles in cell-cell signalling. Most of the human cell surface receptors have been identified in the post-Human Genome Project era but many of their corresponding ligands remain unknown. To facilitate the pairing of orphan receptors, 2762 sequences encoding all human single-pass transmembrane proteins were selected for inclusion into a mammalian-cell expression library. This expression library, consisting of all the individual extracellular domains (ECDs), was constructed as a Fab fusion for each protein. In this format, individual ECD can be produced as a soluble protein or displayed on cell surface, depending on the applied heavy-chain Fab configuration. The unique design of the Fab fusion concept used in the library led to not only superior success rate of protein production, but also versatile applications in various high-throughput screening paradigms including protein-protein binding assays as well as cell binding assays, which were not possible for any other existing expression libraries. The protein library was screened against human coagulation factor VIIa (FVIIa), an approved therapeutic for the treatment of hemophilia, for binding partners by AlphaScreen and ForteBio assays. Two previously known physiological ligands of FVIIa, tissue factor (TF) and endothelial protein C receptor (EPCR) were identified by both assays. The cell surface displayed library was screened against V-domain Ig suppressor of T-cell activation (VISTA), an important immune-checkpoint regulator. Immunoglobulin superfamily member 11 (IgSF11), a potential target for cancer immunotherapy, was identified as a new and previously undescribed binding partner for VISTA. The specificity of the binding was confirmed and validated by both fluorescence-activated cell sorting (FACS) and surface plasmon resonance (SPR) assays in different experimental setups. [ABSTRACT FROM AUTHOR]