The steady state of protein tyrosyl phosphorylation in cells is regulated by the opposing action of tyrosine kinases and protein tyrosine phosphatases (PTPs). Several groups have independently identified a non transmembrane PTP, designated SHPTP1 (also known as PTP1C, HCP and SHP), which is primarily expressed in hematopoietic cells and characterized by the presence of two SH2 domains N terminal to the PTP domain. A second and much more widely expressed PTP with SH2 domains, SHPTP2 (also de