Fmoc-Cys(MMt)-OH is widely used in peptide synthesis, particularly in solid-phase peptide synthesis (SPPS). It serves as a protected cysteine derivative, where the Fmoc group protects the amino group, and the MMt (methoxytrityl) group protects the thiol side chain of cysteine. This dual protection ensures selective deprotection during peptide chain assembly, allowing for precise control over the synthesis process. It is especially valuable in constructing peptides with multiple disulfide bonds, as the MMt group can be selectively removed without affecting other protecting groups. This chemical is also utilized in the synthesis of complex biomolecules, such as proteins and peptide-based drugs, where the precise incorporation of cysteine residues is critical for functionality and stability.