(S)-2-((((9H-Fluoren-9-yl)methoxy)carbonyl)amino)-5-aminopentanoic acid hydrochloride, commonly known as Fmoc-Orn(Boc)-OH or Fmoc-Orn-OH·HCl, is a protected derivative of the non-proteinogenic amino acid ornithine. It features an Fmoc (9-fluorenylmethoxycarbonyl) group protecting the alpha-amino group and a free side-chain amino group (often used as the hydrochloride salt or further protected with a Boc group). This compound is a white to off-white crystalline powder with a molecular formula of C20H22N2O4·HCl. This amino acid derivative is a crucial building block in solid-phase peptide synthesis (SPPS), particularly employing the Fmoc/t-Bu strategy. It is used to incorporate ornithine residues into peptide chains. Ornithine is a key intermediate in the urea cycle and a precursor for polyamines like putrescine. Peptides containing ornithine are of significant interest in biomedical research for developing antimicrobial peptides, enzyme inhibitors, and other bioactive molecules. The Fmoc protection allows for orthogonal deprotection under mild basic conditions, making it compatible with standard SPPS protocols. As a specialized, protected amino acid, its primary and almost exclusive application is in peptide chemistry for research and pharmaceutical development. It is not a commodity chemical or a general-purpose synthesis reagent. Handling requires standard laboratory precautions for fine chemicals.