Receptor-type tyrosine-protein phosphatase kappa is an enzyme that in humans is encoded by the PTPRK gene. PTPRK is also known as PTPkappa and PTPκ. Function The protein encoded by this gene is a member of the protein tyrosine phosphatase (PTP) family. Protein tyrosine phosphatases are protein enzymes that remove phosphate moieties from tyrosine residues on other proteins. Tyrosine kinases are enzymes that add phosphates to tyrosine residues, and are the opposing enzymes to PTPs. PTPs are known to be signaling molecules that regulate a variety of cellular processes including cell growth, differentiation, mitotic cycle, and oncogenic transformation. The human PTPRK gene is located on the long arm of chromosome 6, a putative tumor suppressor region of the genome. During development The same reporter construct used by Shen and colleagues, and described above was created by Skarnes et al. during a screen to identify genes important in mouse development. The transgenic mouse was created by combining a β-galactosidase (β-gal) reporter gene with a signal sequence and the transmembrane domain of the type I transmembrane protein CD4. If the transgene was incorporated into a gene with a signal sequence, β-gal activity would remain in the cytosol of the cell and therefore be active. If the reporter gene was incorporated into a gene that lacked a signal sequence, β-gal activity would be in the ER where it would lose β-gal activity. This construct inserted into the phosphatase domain of PTPkappa. Mice generated from these ES cells were viable, suggesting that PTPkappa phosphatase activity is not necessary for embryonic development. Additional studies have suggested a function for PTPkappa during nervous system development. PTPkappa promotes neurite outgrowth from embryonic cerebellar neurons, and thus may be involved in axonal extension or guidance in vivo. Neurites are extensions from neurons that can be considered the in vitro equivalent of axons and dendrites. The extension of cerebellar neurites on purified PTPkappa fusion proteins was demonstrated to require Grb2 and MEK1 activity. In T cells PTPkappa has also been shown to regulate CD4+ positive T cell development. PTPkappa and the THEMIS gene are both deleted in the rat Long-Evans Cinnamon (LEC) strain, and are both required for the CD4+ T-cell deficiency observed in this strain of rats. Deletion of PTPkappa was shown to generate T-helper immunodeficiency in the LEC strain. By expressing a dominant negative form of PTPkappa or by using short-hairpin RNA for PTPkappa in bone-marrow derived stem