Lectins are a kind of carbohydrate-binding proteins. Though they differ in their carbohydrate specificities, they resemble each other in many physicochemical properties. By now, a lot of plant lectins have been sequenced, and some of their three-dimensional structures have been established. A few lectin genes have been cloned. Comparison of their primary sequences and tertiary structures, we can find that plant lectins are several large groups of homologous proteins belonging to a superfamily.