Genes in somatic cell tissues are normally expressed only when their products are necessary for functional purposes or structural repair. They may be activated into expression, and then again switched off, through the action of other molecules expressed by other genes, or they may be artificially induced into expression, whether for gene therapeutic or experimental goals. The process of gene activation is termed "gene induction." Genes induced into expression transcribe the coded information to the synthesis of a functional molecule, such as, RNA molecules, enzymes, hormones, growth factors, etc. Exposure to ultra violet solar radiation, for example, induces the expression of genes in eukaryotic cells that encode enzymes involved in DNA photo-damage repair (i.e., repair of damages caused to DNA by sunlight exposure).

Gene expression controls are many and varied, some of them being tissue-specific, whereas others may have a pleiotropic effect. A gene or...