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“small molecule” is often used to refer to things like traditional pharmaceutical drugs, but “small molecules” are also naturally made & used in the body, such as ATP. It’s basically just a classification for “low molecular weight” (i.e. small) molecules (typically organic (hydrocarbon-based) ones). So you could use “small molecule” to describe ATP, but if that ATP is used as an RNA letter to form a big ole chain of RNA, that RNA is a called a macromolecule (“big molecule”). Proteins are not small molecules, but proteins can bind small molecules and the small molecules can affect their function.