(Fmoc-Cys-OtBu)₂ is a symmetric disulfide dimer of N-Fmoc-protected L-cysteine with tert-butyl esters on the carboxylic acid groups. It serves as a key building block in solid-phase peptide synthesis (SPPS) for incorporating cystine residues, enabling the direct introduction of disulfide bonds in peptide chains. The Fmoc groups allow orthogonal deprotection of the α-amino functions under mild basic conditions, while the pre-formed disulfide bridge protects the thiol side chains from side reactions during coupling and assembly. The tBu esters safeguard the C-terminal carboxyl groups, which can be removed later under acidic conditions. This specialty reagent is vital for constructing complex peptides and proteins with precise disulfide connectivity, supporting advanced research in biochemistry, structural biology, and pharmaceutical development.