Plexin B1 is a 300 kDa member of the B subfamily, Plexin family of Semaphorin receptors. Mature human Plexin B1 is a 2116 amino acid type I transmembrane (TM) glycoprotein that contains a 1471 aa extracellular domain (ECD) and a 612 aa cytoplasmic region. The ECD contains one Semaphorin domain, two PSI domains, and three IPT repeats. The ECD is cleaved into two subunits, a 200 kDa a-chain (aa 20-1305) and a 100 kDa TM b-chain. The subunits are nondisulfide-linked and generate a high-affinity receptor. Plexin B1 is a receptor for Semaphorin 4D/CD100. It forms a receptor complex with Neuropilins, MET, and EGF R2. Multiple splice variants are known. One shows a deletion of aa 689-871. A second shows a 53 aa substitution for aa 677-2135. And a third shows a 53 aa substitution for the 53 aa between aa 677-729. Over aa 35-150, human Plexin B1 is 90% and 97% aa identical to dog and mouse Plexin B1, respectively.