2-Hydroxy-5-(methylthio)-benzoic Acid-D3 is an isotopic labelled analog of 2-Hydroxy-5-(methylthio)-benzoic Acid. 2-Hydroxy-5-(methylthio)-benzoic acid is used in biological studies to evaluate the pharmcological activity and the therapeutic use in fragment based discovery of potent inhibitors of anti-apoptotic MCL-1 protein.