Whereas aldosterone is normally a much stronger mineralocorticoid than cortisol in vivo, mineralocorticoid receptors have identical in vitro affinities for these hormones. The in vivo specificity of the receptors is, at least in part, the result of activity of 11-HSD, an enzyme located in most mineralocorticoid target tissues that converts cortisol to cortisone. Cortisone is not a ligand for the receptor, whereas aldosterone is not a substrate of the enzyme. The syndrome of AME is a rare form of juvenile hypertension in which 11-HSD is defective. This deficiency allows mineralocorticoid receptors to be occupied by cortisol, leading to hypertension, because plasma concentrations of cortisol are much higher than those of aldosterone. Licorice, which contains 11-HSD inhibitors, causes a similar syndrome. There are two known isozymes of 11-HSD. The liver or type I isozyme is expressed at high levels in the liver, has a relatively low affinity for steroids (micromolar Km), catalyzes both dehydrogenation and the reverse reductase reaction, and utilizes NADP+ or NADPH as cofactors. The kidney or type 2 isozyme is expressed at high levels in the kidney and placenta, has a high affinity (nanomolar Km) for steroids, catalyzes only dehydrogenation, and utilizes NAD+ as a cofactor. Mutations in the HSD11B2 (HSD11K) gene encoding the kidney isozyme of 11-HSD have been detected in all kindreds with AME studied thus far. This gene represents a candidate locus for the common, "essential" form of hypertension.