Staphylococcal Sortase A is a bacterial transpeptidase that covalently attaches proteins to the bacterial cell wall by cleaving between threonine and glycine at an LPXTG recognition motif and catalyzes the formation of an amide bond between the carboxyl-group of threonine and the amino-group of the cell-wall peptidoglycan. This chemistry has been exploited as a molecular "stapler" to site-specifically link proteins with the C-terminal LPETGX motif to other proteins or molecules possessing a glycine or aminomethylene motif. This reaction is applicable to proteins in solution and on the living cell surface.