Phenylthiohydantoin-(Nε-phenylthiocarbamyl)-lysine is primarily utilized in the field of protein sequencing and peptide analysis. It serves as a derivative in the Edman degradation process, a method used to determine the amino acid sequence in proteins. By reacting with the N-terminal amino acid of a peptide, it forms a stable phenylthiohydantoin derivative, which can be easily identified through chromatographic techniques. This allows for the sequential removal and identification of amino acids, aiding in the structural analysis of proteins. Additionally, it is employed in biochemical research to study protein modifications and interactions, providing insights into protein function and behavior in various biological systems.