Introduction . The extracellular proteome, which consists of secreted and membrane-bound proteins, is encoded by at least a quarter of the human genome (4,000–5,000 genes)1. This tremendous genetic investment is not surprising given that this proteome represents the hub for cells and tissues to communicate externally. Moreover, half of the FDA-approved small-molecule drugs engage this proteome, as do virtually all biologics — especially antibodies, which currently represent about h...