Under standard conditions, chlorine exists as a diatomic molecule. Chlorine is a highly toxic, pale yellow-green gas that has a specific strong smell. In nature, chlorine is most abundant as a chloride ion. Physiologically, it exists as an ion in the body. The chloride ion is an essential anion that the body needs for many critical functions. It also helps keep the body's acid-base balance. The amount of chloride in the blood is carefully controlled by the kidneys. Chloride ions have important physiological roles. For instance, in the central nervous system, the inhibitory action of glycine and some of the action of GABA relies on the entry of Cl- into specific neurons. Also, the chloride-bicarbonate exchanger biological transport protein relies on the chloride ion to increase the blood's capacity of carbon dioxide, in the form of the bicarbonate ion. Chloride-transporting proteins (CLC) play fundamental roles in many tissues in the plasma membrane as well as in intracellular membranes. CLC proteins form a gene family that comprises nine members in mammals, at least four of which are involved in human genetic diseases. GABA(A) receptors are pentameric complexes that function as ligand-gated chloride ion channels. WNK kinases are a family of serine-threonine kinases that have been shown to play an essential role in the regulation of electrolyte homeostasis, and they are found in diverse epithelia throughout the body that are involved in chloride ion flux. Cystic fibrosis (CF) is caused by alterations in the CF transmembrane conductance regulator (CFTCR) gene that result in deranged sodium and chloride ion transport channels. (For a complete review see Evans, Richard B. Chlorine: state of the art. Lung (2005), 183(3), 151-167. PMID: 16078037).