Protein tyrosine-protein phosphatase required for T-cell activation through the antigen receptor. Acts as a positive regulator of T-cell coactivation upon binding to DPP4. The first PTPase domain has enzymatic activity, while the second one seems to affect the substrate specificity of the first one. Upon T-cell activation, recruits and dephosphorylates SKAP1 and FYN (By similarity). Dephosphorylates LYN, and thereby modulates LYN activity (By similarity). H2O + O-phospho-L-tyrosyl-[protein] = L-tyrosyl-[protein] + phosphate Interacts with SKAP1. Interacts with DPP4; the interaction is enhanced in an interleukin-12-dependent manner in activated lymphocytes. Binds GANAB and PRKCSH (By similarity). Colocalized with DPP4 in membrane rafts. Additional isoforms seem to exist. Isoform 1 and isoform 2 are found in thymocyte and lymph node. Isoform 4 and isoform 3 are found in the lymph nod. The first PTPase domain interacts with SKAP1. Heavily N- and O-glycosylated. The cytoplasmic domain contains potential phosphorylation sites. Belongs to the protein-tyrosine phosphatase family. Receptor class 1/6 subfamily.