Type IIa receptor protein tyrosine phosphatases (RPTPs) are a group of well-characterized proteins that are involved in axon growth and guidance during neural development. Members of this subfamily, RPTPσ, RPTPδ and LAR contain two cytoplasmic phosphatase domains and extracellular immunoglobulin domains followed by fibronectin type III repeats. RPTPσ is a type I transmembrane protein that has an apparent molecular weight of approximately 80 kDa. It is highly expressed by neurons in developing and adult mammalian nervous system. Knock-out mice for RPTPσ show growth retardation, delayed peripheral nerve development and high mortality rates. The role of RPTPσ in axon guidance during development suggests it could influence axon regeneration after injury in the adult mouse. It has been shown that RPTPσ slows axon regeneration in the adult injured CNS.