SHP-1 is mainly expressed in blood cells.[34] It contains two SH2 domains and a catalytic domain (the region of a protein that carries out the main function of the protein) - the catalytic domain contains the amino acid sequence VHCSAGIGRTG (a sequence typical of PTPs).[35] As with all PTPs, a number of amino acid structures are essential for their function: conserved cysteine, arginine and glutamine amino acids, and a loop made of tryptophan, proline and aspartate amino acids (WPD loop).[35] When SHP-1 is inactive, the SH2 domains interact with the catalytic domain, and so the phosphatase is unable to function.[35] When SHP-1 is activated however, the SH2 domains move away from the catalytic domain, exposing the catalytic site and therefore allowing phosphatase activity.[35] SHP-1 is then able to bind and remove phosphate groups from the JAKs associated with receptors, preventing the transphosphorylation needed for the signalling pathway to progress.