S-tag is the name of an oligopeptide derived from pancreatic ribonuclease A. The amino acid sequence of the S-tag is: Lys-Glu-Thr-Ala-Ala-Ala-Lys-Phe-Glu-Arg-Gln-His-Met-Asp-Ser. It is believed that the peptide with its abundance of charged and polar residues could improve solubility of proteins it is attached to. This antibody recognizes the s-tag fused to either the amino- or carboxy-terminus of targeted proteins.