Or ALDH, this breaks down various aldehydes into the corresponding carboxylic acids. Its most notable substrate is, of course ethanal the breakdown product of ethanol. It is the lack of this crucial protein that causes alcohol intolerance in 25% of the Asian population. Buildup of the ethanal intermediate causes toxic effects similar to drinking far too much. This is (not coincidentally) the basis of Anabutase action - the drug given to combat alcoholism. It blocks ALDH, preventing users from drinking more than a tiny amount.

The ALDH family is large and covers a large range of substrates, but another notable one is retinal (a vitamin A derivative) which is made into retinoic acid. This is a morphogen that can alter the developmental state of mammalian tissue. RALDH's may, therefore, be involved in the patterning and growth of the embryo.