Phenylthiohydantoin-leucine is primarily used in the field of protein sequencing and peptide analysis. It serves as a derivative in the Edman degradation process, a method for determining the amino acid sequence of proteins. By selectively cleaving one amino acid at a time from the N-terminus of a peptide, it allows for the identification of leucine residues within the sequence. This compound is particularly valuable in biochemical research and pharmaceutical development, where understanding protein structure is crucial for drug design and the study of biological processes. Its application is essential in laboratories focused on proteomics and molecular biology.