Protein phosphatases play critical roles in the regulation of signal transduction pathways. The family can be separated into three general categories, which are distinguished on the basis of substrate specificity. The first and largest category, termed protein tyrosine phosphatases or PTPs, includes transmembrane proteins, membrane associated proteins and proteins that localize to nuclei. The second category of protein phosphatases dephosphorylate proteins on phosphoserine and phosphothreonine residues, whereas the third category of protein phosphatases exhibit dual specificities and can dephosphorylate proteins on phosphotyrosine and phosphoserine/phosphothreonine residues. Glomerular epithelial protein 1 (GLEPP1), also designated protein tyrosine phosphatase U2 (PTPase U2) or receptor-type tyrosine-protein phosphatase O (PTPRO), belongs to the protein-tyrosine phosphatase family. GLEPP1 is a Type I membrane protein containing 8 fibronectin type-III domains and 1 tyrosine-protein phosphatase domain. GLEPP1 is expressed primarily in the glomeruli of the kidney, but is also detected in placenta, lung and brain.