The PB1 domain is present in the Epichloe festucae NoxR protein (NADPH oxidase regulator), a key regulator of NADPH oxidase isoform, NoxA. NoxA is essential for growth control of the fungal endophyte in plant tissue in the process of symbiotic interaction between a fungi and its plant host. The Epichloe festucae p67(phox)-like regulator, NoxR, dispensable in culture but essential in plants for the symbiotic interaction. Plants infected with a noxR deletion mutant show severe stunting and premature senescence, whereas hyphae in the meristematic tissues show increased branching leading to increased fungal colonization of pseudostem and leaf blade tissue. The PB1 domain is a modular domain mediating specific protein-protein interactions which a play role 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.