The first critical step in conducting candidate gene studies is the choice of a suitable candidate gene that may plausibly play a relevant role in the process or disease under investigation. For example, when studying alcoholism, genes encoding enzymes that act in various pathways of alcohol metabolism, such as alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH), are logical choices. Both enzymes are encoded by more than one gene (i.e., by gene families), and each of these genes exists in several variants, or alleles, allowing for its use in the candidate gene approach.
