From the hydropathic profiles of their amino acid sequences many transport proteins are conceived to comprise 12-transmembrane alpha-helices. In only a few examples, however, is there genetical and/or biochemical evidence to support the 12-helix structure or illuminate the molecular mechanism of the transport process. A number of these transport proteins occur in evolutionarily related families, and sometimes superfamilies, indicating divergent evolution of the 12-helix structure. Other individual members or families of transport proteins are sufficiently different in amino acid sequence for their evolution to have taken place by convergence from independent ancestral origins.