The definition above excludes protein complexes, which some also consider a protein. Those who wish to refer to a class representing both senses of the word are directed to CHEBI:36080. Proteins (in the sense defined here) that descended from a common ancestor can be classified into families and superfamilies composed of products of evolutionarily-related genes. The domain architecture of a protein is described by the order of its constituent domains. Proteins with the same domains in the same order are defined as homeomorphic [PRO:WCB].