n. (Medicine, Biochemistry) The mechanism{ 2 } by which a pharmacologically active substance produces an effect on a living organism or in a biochemical system; as, the mechanism of action of actinomycin involves its binding to DNA. The mechanism of action is usually considered to include an identification of the specific molecular targets to which a pharmacologically active substance binds or whose biochemical action it influences; a general recognition of the broad biochemical pathways (such as DNA synthesis, protein synthesis, cholesterol synthesis) which are inhibited or affected by a substance is termed its mode of action. [ PJC ]