Fmoc-DL-Nle-OH is a protected amino acid derivative, specifically the racemic mixture of N-fluorenylmethoxycarbonyl (Fmoc) protected norleucine. Norleucine (Nle) is a non-proteinogenic, aliphatic amino acid analog of methionine, where the sulfur atom is replaced by a methylene group. The Fmoc group serves as a base-labile protecting group for the α-amino function, making this compound a crucial building block in solid-phase peptide synthesis (SPPS) using the Fmoc strategy. Its primary application is in the field of peptide chemistry and life sciences research. As a synthetic amino acid derivative, it is used to incorporate norleucine residues into custom peptide sequences. Norleucine is often employed as an isosteric replacement for methionine to study structure-activity relationships, enhance metabolic stability, or prevent oxidation issues associated with methionine. It is also used as an internal standard in amino acid analysis and mass spectrometry due to its similar properties to methionine but distinct mass. This reagent is a staple in laboratories focused on peptide drug discovery, biochemical tool development, and proteomics. It is commercially available from major suppliers of peptide synthesis reagents and fine chemicals. Standard precautions for handling fine chemicals should be observed.