  A combination of DNA segments that together constitute an expressible unit, expression leading the formation of one or more specific functional gene products that may lead to either RNA molecules or polypeptides. The segments of a gene include (1) the transcribed unit … and any regulatory segments included in the transcription unit, and (2) the regulatory sequences that flank the trancription unit and are required for specific expression. (Singer and Berg 1991, p. 41).