CD177 is polymorphic and has at least two alleles: PRV1 and NB1. Human PRV1 is a Glycosyl-Phosphatidylinositol (GPI)-linked cell surface glycoprotein that belongs to the uPAR/CD59/Ly6 family of receptors. PRV1 is expressed by neutrophils and neutrophil precursors,and changes in expression serve as diagnostic markers for myeloproliferative disorders such as polycythemia vera and essential thrombocythemia. PRV1 may also be expressed by Erythroblasts, B cells, and Monocytes. NB1, a Glycosyl-Phosphatidylinositol (GPI)-linked cell surface glycoprotein, was first described in a case of neonatal alloimmune neutropenia. It is reported that CD177 functions as a novel heterophilic binding partner that engages PECAM-1 in membrane-proximal IgD6.