Edman degradation, developed by Pehr Edman , is a method of sequencing amino acids in a peptide. In this method, the amino-terminal residue is labeled and cleaved from the peptide without disrupting other peptide bonds between other amino acid residues. Phenyl isothiocyanate is reacted with uncharged terminal amino group to form a phenylthiocarbamoyl derivative. Then, under mildly acidic conditions, this derivative of the terminal amino acid is cleaved. The derivative that was cleaved is known as phenylthiohydantoin (PTH)- amino acid that can be identified by using chromatography. This procedure can then be repeated again to identify the next amino acid. A major drawback to this technique is that the peptides being sequenced in this manner cannot have more than 50 to 60 residues. This is because the Edman degradation reaction is not 100% efficient, meaning that the cleavage step does not occur every time. However, this problem can be resolved by cleaving large peptides into smaller peptides before proceeding with the reaction.