Six proteins have so far been isolated that to the higher primates. have an intensely sweet taste: monellin, thaumatin, brazzein, pentadin, mabinlin, and curculin. Curculin also acts as a taste-modifying protein, as does the protein miraculin. These proteins are all isolated from plants, but span a wide range of molecular weights and biochemical properties. Sequence comparisons show little similarity between the sweet proteins and suggest that they evolved independently of each other and are descendants of other proteins which wan originally parts of the molecular machinery of the plant. Structures haw been determined for several of the sweet proteins but these structures bear no resemblance to each other. Various method have been used to identify residues in these proteins that are responsible for their sweet taste. Thus far no common motif for a sweet determinant has been identified, although come immunological studies suggest that such a common structure may exist.