Structurally, all known receptor PTPases, are made up of a variable length extracellular domain, followed by a transmembrane region and a C-terminal catalytic cytoplasmic domain. Some of the receptor PTPases contain fibronectin type III (FN-III) repeats, immunoglobulin-like domains, MAM domains or carbonic anhydrase-like domains in their extracellular region. The cytoplasmic region generally contains two copies of the PTPase domain. The first seems to have enzymatic activity, while the second is inactive. The inactive domains of tandem phosphatases can be divided into two classes. Those which bind phosphorylated tyrosine residues may recruit multi-phosphorylated substrates for the adjacent active domains and are more conserved, while the other class have accumulated several variable amino acid substitutions and have a complete loss of tyrosine binding capability. The second class shows a release of evolutionary constraint for the sites around the catalytic centre, which emphasises a difference in function from the first group. There is a region of higher conservation common to both classes, suggesting a new regulatory centre [PMID: 14739250]. PTPase domains consist of about 300 amino acids. There are two conserved cysteines, the second one has been shown to be absolutely required for activity. Furthermore, a number of conserved residues in its immediate vicinity have also been shown to be important.