Animal lectins display a wide variety of architectures. They are classified according to the carbohydrate-recognition domain (CRD) of which there are two main types, S-type and C-type. Galectins (previously S-lectins) bind exclusively beta-galactosides like lactose. They do not require metal ions for activity. Galectins are found predominantly, but not exclusively in mammals. Their function is unclear. They are developmentally regulated and may be involved in differentiation, cellular regulation and tissue construction.