Cluster of Differentiation 100 (CD100) is also known as Semaphorin-4D (SEMA4D), is a single-pass type I membrane protein which belongs to the semaphorin family, and is a human 150-kDa homodimer expressed at the surface of most hemopoietic cells. Semaphorin genes encode soluble and membrane-bound proteins, most of which have been shown to act as chemorepellents on growth cone guidance. CD100 is discrete, as it is a transmembrane leukocyte surface molecule that can also exist in a soluble form. Semaphorin 4D (Sema 4D) is an axon guidance molecule which is secreted by oligodendrocytes and induces growth cone collapse in the central nervous system. By binding plexin B1 receptor it functions as an R-Ras GTPase-activating protein (GAP) and repels axon growth cones in both the mature central nervous system. In the immune system, CD100 binds CD72 to activate B cells and dendritic cells, though much about this interaction is still under investigation. It is involved in oligodendrogenesis during development and during recovery from ischemic injury.