Crystal structures of integral membrane proteins; (A) Superimposed known structures of GPCRs including invertebrate rhodopsin: The characteristic seven trans membrane helices and succeeding short helix 8 (7TMH+H8) are conserved among all structure known GPCRs. (B) Structures of MAPEG families including leukotriene C4 synthase: There are three active sites between adjacent monomers in the trimer of four transmembrane helices monomers.