Introduction: Transporters, Porins, and Efflux Pumps.

All living cells are surrounded by lipidic membranes that separate metabolic and macromolecular biosynthetic processes from external environments. Biological membranes vary dramatically in their composition and structures and are optimized by mega-annum of evolution to effectively carry out diverse biological functions including energy production, biosynthetic reactions, signaling, uptake of nutrients and active efflux, and others. This thematic issue of Chemical Reviews, "Transporters, Porins, and Efflux Pumps", is focused on the function of biological membranes as a permeability barrier and on the proteins that create, maintain, and bypass this barrier. Effective therapeutic interventions rely on active and passive permeation of various molecules and compounds across membranes, and our successes in development of such therapeutics are strongly affected by structural and functional insights into the assembly and function of cellular permeation barriers.