Protein tags are protein or peptide sequences located either on the C- or N- terminal of the target protein, which facilitates one or several of the following characteristics: solubility, detection, purification, localization and expression. The S tag is corresponds to amino acid residues KETAAAKFERQHMDS of pancreatic ribonuclease A (RNase A). Many recombinant proteins have been engineered to express the S tag. It is believed that the peptide with its abundance of charged and polar residues could improve solubility of proteins it is attached to. Moreover, the peptide alone is thought not to fold into a distinct structure. This tag facilitates the detection, isolation, and purification of the proteins.