(2S)-2-Amino-4-hexynoic acid, commonly known as L-Homopropargylglycine or L-HPG, is a non-canonical, alkyne-containing amino acid analog of methionine. Its structure features a terminal alkyne group on the side chain, which serves as a bioorthogonal chemical reporter. This compound is typically supplied as a white to off-white powder. The primary application of L-HPG is in chemical biology and proteomics research. It is efficiently incorporated into newly synthesized proteins by the cell's endogenous translational machinery in place of methionine. The alkyne handle allows for subsequent, highly specific detection and enrichment via copper-catalyzed azide-alkyne cycloaddition (CuAAC) or strain-promoted azide-alkyne cycloaddition (SPAAC) reactions with azide-functionalized probes (e.g., fluorescent dyes or biotin). This enables visualization, quantification, and isolation of nascent proteins, making it a powerful tool for studying protein synthesis dynamics, turnover, and localization in living cells. Due to its role as a fundamental building block for bioorthogonal labeling, it also finds use as a specialized synthetic intermediate in the development of probes and other research reagents.