Background：

Semaphorins are a large family of conserved secreted and membrane associated proteins which possess a semaphorin (Sema) domain and a PSI domain (found in plexins， semaphorins and integrins) in the N-terminal extracellular portion. Based on sequence and structural similarities， semaphorins are put into eight classes: invertebrates contain classes 1 and 2， viruses have class V， and vertebrates contain classes 3-7. Semaphorins serve as axon guidance ligands via multimeric receptor complexes， some (if not all) containing plexin proteins. Multiple transcript variants encoding different isoforms have been found for this gene.