Fmoc-Cys(Et)-OH is widely used in peptide synthesis as a protecting group for cysteine residues. The Fmoc group shields the amino group during peptide chain assembly, while the ethyl (Et) group protects the thiol side chain of cysteine, preventing unwanted side reactions or oxidation. This compound is particularly valuable in solid-phase peptide synthesis (SPPS), where selective deprotection allows for controlled peptide elongation. Its application ensures the stability and integrity of cysteine-containing peptides, which are often prone to disulfide bond formation or degradation. This makes it essential for producing peptides used in drug development, biochemical research, and the study of protein interactions.