Anchored periplasmic expression (APEx) is a method for isolating high affinity ligand-binding proteins from large combinatorial libraries, and antibodies highly specific for soluble antigens were successfully isolated from APEx antibody libraries in combination with flow cytometric sorting.However, many disease markers and drug targets are localized on the cell surface, and often unique posttranslational modifications and/or properly folded epitopes are lost when they were expressed and isolated in soluble form.