Semaphorins are a family of cell surface and secreted proteins that are conserved from insects to humans. Members of this family of proteins are approximately 750 amino acids in length (including signal sequences) and are defined by a conserved extracellular “semaphorin” domain of approximately 500 amino acids containing 14-16 cysteines, blocks of conserved sequences and no obvious repeats. Secreted and cell-bound semaphorins chemically attract and repel the growth of neural axons, guiding the development of intricate networks of neural tissue. SEMA4G (semaphorin-4G) is an 838 amino acid single-pass type I membrane protein that is thought to play a role in axon guidance. Existing as three alternatively spliced isoforms, SEMA4G contains one Ig-like C2-type (immunoglobulin-like) domain, a PSI domain and a single SEMA domain.