2-Amino-3-mercaptopropionic acid methyl ester, also known as methyl 2-amino-3-mercaptopropanoate, is a derivative of the amino acid cysteine. It features both an amino group and a thiol (mercapto) group, making it a versatile bifunctional building block. The compound is typically a white to off-white crystalline powder. This compound is primarily used as a key intermediate in the synthesis of various pharmaceuticals and bioactive molecules. Its structure allows it to serve as a precursor for cysteine-containing peptides and peptidomimetics. It is also employed in the preparation of compounds with potential therapeutic applications, particularly those targeting conditions where thiol chemistry is relevant. As a reagent, it is valued for its reactivity in nucleophilic substitution and addition reactions due to the thiol group, and in condensation reactions via the amino group. Proper handling requires consideration of its potential sensitivity to oxidation and standard precautions for thiol-containing compounds.