Ac-Cys(Trt)-OH is used primarily in peptide synthesis as a doubly protected form of cysteine. The N-terminus is acetylated (Ac) for permanent protection, while the thiol group is shielded by the trityl (Trt) group to prevent unwanted side reactions during chain assembly. This protection strategy is essential in solid-phase peptide synthesis, enabling selective deprotection of the Trt group for controlled disulfide bond formation, which is critical for the proper folding and bioactivity of peptides and proteins. It is widely applied in the synthesis of therapeutic peptides, research peptides, and studies of protein structures.