Abstract: Genetic markers of this invention provide a biochemically assayable reporter activity and a conditionally selectable growth advantage. They comprise a first gene which encodes a product whose activity can be assdyed biochemically e.g., .beta.-glucuronidase, to a second gene which confers a conditional growth advantage such as antibiotic resistance in a transformed cell (e.g., neomycin phosphotransferase-II). The marker genes and their translational products are stable in cells. The genetic markers provide for the positive genetic selection and the biochemical and histochemical screening of transformed cells. They can be used as probes for identification and isolation of novel genetic regulatory elements, such as promoters, and the genes they regulate.