A cistron is a region of DNA that is conceptually equivalent to some definitions of a gene, such that the terms are synonymous from certain viewpoints, especially with regard to the molecular gene as contrasted with the Mendelian gene. The question of which scope of a subset of DNA constitutes a unit of selection is the question that governs whether cistrons are the same thing as genes. The word cistron is used to emphasize that molecular genes exhibit a specific behavior in a complementation test ; distinct positions within a genome are cistronic.