KETAAAKFERQHMDS tag,S peptide,S peptide epitope tag,S tag is an epitope tag composed of a 15 residue peptide, KETAAAKFERQHMDS, derived from the pancreatic ribonuclease A. This interactive complex (S-tag/S-protein) results in micromolar affinity - dependent upon pH , temperature, and ionic strength . The S-tag itself enables colorimetric detection via western blot analysis or a rapidly detected assay dependent upon ribonucleolytic reconstitution and activity.