The PB1 domain is present in the p51 protein, a homolog of the p67 protein. p51 plays an important role in NADPH oxidase activation during phagosytosis. The PB1 domain is a modular domain mediating specific protein-protein interaction in many critical cell processes such as osteoclastogenesis, angiogenesis, early cardiovascular development, and cell polarity. A canonical PB1-PB1 interaction, which involves heterodimerization of two PB1 domains, is required for the formation of macromolecular signaling complexes ensuring specificity and fidelity during cellular signaling. The interaction between two PB1 domain depends on the type of PB1. There are three types of PB1 domains: type I which contains an OPCA motif, acidic aminoacid cluster, type II which contains a basic cluster, and type I/II which contains both an OPCA motif and a basic cluster. Interactions of PB1 domains with other protein domains have been described as noncanonical PB1-interactions. The PB1 domain module is conserved in amoebas, fungi, animals, and plants.