Int. J. Environ. Res. Public Health 2010, 7 1092 sequences for the three ADH1 isozymes are about 91% homologous [77]. On the other hand, ADH2 (formerly ADH4), 3 (formerly ADH5), and 4 (formerly ADH7) contain a homodimer of subunit π, χ, and δ (μ), respectively. The amino acid sequences of ADH 1, 2, 3, and 4 are about 60% homologous [77]. The Km for ethanol increases in the order ADH 1, 2, 4, 3, with the values being around 1 mM, several tens of millimoles, several hundred millimoles, and several moles, respectively [13,16]. The insensitivity to pyrazoles, which are specific inhibitors of ADH, also increases in the same order [13,16]. Thus, among mammalian ADH isozymes, ADH3 has the highest Km for ethanol and the lowest sensitivity to pyrazoles, while ADH1 has the lowest Km and the highest sensitivity. Unlike other ADHs, which have limited tissue distributions, ADH3 is present in almost all mammalian tissues. Moreover, ADH3 exists in all biological kingdoms, while other mammalian ADHs appear first in vertebrates. So, ADH3 is considered to be the ancestor of all other ADHs [68]. © 2010 by the authors; licensee Molecular Diversity Preservation International, Basel, Switzerland. This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).