Soluble PROCR binds to activated neutrophils via PRTN3, also cknown as myeloblastin and (Leukocyte) proteinase-3 (Kurosawa et al. 2000). PRTN3 is the most abundant serine protease in neutrophils (Campbell et al. 2000). After neutrophil activation, PRTN3 is secreted from azurophil granules, rebinding to the neutrophil surface through an association with CD177 (NB1) a 60-kDa glycosyl-phosphatidylinositol (GPI)-linked cell surface glycoprotein, which is expressed on a subpopulation of neutrophils in 97% of healthy individuals (Knuckleburg et al. 2012). PRTN3 is partially protected from inactivation when associated with CD177 (Campbell et al. 2000) which may increase its efficacy. CD177 is a heterophilic binding partner for endothelial cell platelet-endothelial cell adhesion molecule (PECAM)-1, which is expressed at endothelial cell junctions where transmigration occurs (Sun et al. 2000) suggesting that CD177 directs at least a subpopulation of PRTN3 molecules to these areas to aid neutrophil diapedesis, perhaps through PRTN3 degradation of cell junction proteins or the extracellular matrix.