Fig. 2. The semaphorin family of proteins. The known members of the semaphorin family have been categorised into 8 classes (V-7). All semaphorins share a 500 amino acid semaphorin (Sema) domain, which is followed, in some classes, by a single Ig-like domain. Several members of the semaphorin family are secreted molecules with no membrane attachment site (for example Class 2 and Class 3 semaphorins), whereas others are linked to the cell surface by a transmembrane domain or by a GPI anchor. One subfamily, the Class 5 semaphorins, contains a set of thrombospondin type I repeats. Adapted, with permission, from the Semaphorin Nomenclature Committee (Semaphorin Nomenclature Committee, 1999).