Used in the synthesis of antibody-drug conjugates (ADCs), particularly as a linker-payload component. The PAB (para-aminobenzyloxycarbonyl) spacer enables controlled release of cytotoxic agents inside target cells through enzymatic cleavage, typically by cathepsin B. Fmoc-Cit-PAB-OH serves as a protected building block where the citrulline (Cit) residue provides a site for attachment and the Fmoc group allows for solid-phase peptide synthesis compatibility. Its main role is to bridge the antibody and the drug, ensuring stability in circulation and efficient drug release upon internalization into cancer cells.