Ephrins are the ligands which bind to the Eph receptor subfamily, the largest group of the receptor tyrosine kinase family. Both ephrins and Eph receptors are broadly expressed throughout the ectoderm, mesoderm, and endoderm of vertebrate embryos. Two classes of ephrins exist: Class A ephrins are tethered to the membrane by a GPI linkage and bind primarily to EphA receptors; Class B ephrins contain a membrane-spanning region and bind predominantly to EphB receptors.