N-terminal sequencing by Edman degradation, is a traditional method for sequencing protein, and still has advantages for protein analysis that cannot readily be obtained by other analysis methods. Edman degradation is a cyclic procedure: N-terminal amino acid residues is labeled and cleaved off from the peptides or proteins at a time, and is identified by chromatography. Generally, the N-terminal amino group of the protein is reacted with phenyl isothiocyanate to form a phenylthiocarbamoyl derivative. Under mildly acidic conditions, the derivative is released from the rest of the protein as a cyclic phenylthiohydantoin (PTH) amino acid. The released PTH amino acid is identified by high performance liquid chromatography (HPLC) or HPLC-MS. The remainder of the peptide is intact and available for another round of labeling and release.