Cistron

A cistron is a region of DNA that is quite similar to a gene, so much so that the terms are considered synonymous by some, especially with regard to the molecular gene as contrasted with the Mendelian gene. The debate whether or not a cistron is the same thing as a gene, revolves around what size a segment of DNA constitutes as a unit of selection. The word cistron is used to emphasize that molecular genes exhibit a specific behavior in a complementation test (cis-trans test); distinct positions (or loci) within a genome are cistronic.