Semaphorins are members of a large family of proteins controlling a variety of processes in the central nervous system (CNS) during development such as cell migration and axonal growth cone guidance but are also expressed in other tissues. Their common feature is a conserved 500 amino acid “sema” domain. There are currently eight defined classes of semaphorins; classes 2 and 3 are secreted proteins while others are GPI-linked or transmembrane proteins.