The solute-carrier gene (SLC) superfamily encodes membrane-bound transporters. The SLC superfamily comprises 55 gene families having at least 362 putatively functional protein-coding genes. The gene products include passive transporters, symporters and antiporters, located in all cellular and organelle membranes, except, perhaps, the nuclear membrane. Transport substrates include amino acids and oligopeptides, glucose and other sugars, inorganic cations and anions (H(+), HCO(3)(-), Cl(-), Na(+), K(+), Ca(2+), Mg(2+), PO(4)(3-), HPO(4)(2-), H(2)PO(4)(-), SO(4)(2-), C(2)O(4)(2-), OH(-), CO(3)(2-)), bile salts, carboxylate and other organic anions, acetyl coenzyme A, essential metals, biogenic amines, neurotransmitters, vitamins, fatty acids and lipids, nucleosides, ammonium, choline, thyroid hormone and urea. Contrary to gene nomenclature commonly assigned on the basis of evolutionary divergence (http://www.genenames.org/), the SLC gene superfamily has been named based largely on transporter function by proteins having multiple transmembrane domains. Whereas all the transporters exist for endogenous substrates, it is likely that drugs, non-essential metals and many other environmental toxicants are able to 'hitch-hike' on one or another of these transporters, thereby enabling these moieties to enter (or leave) the cell. Understanding and characterising the functions of these transporters is relevant to medicine, genetics, developmental biology, pharmacology and cancer chemotherapy.