Schematic of the binary expression vector. a. In one application of the Cre-loxP system, Cre-mediated recombination between two parallel loxP sites results in excision of the intervening sequence. b. When two loxP sequences are anti-parallel, Cre-mediated recombination results in inversion of the intervening sequence. c. The binary expression vector backbone. Note the antiparallel loxP sites, with a transcriptional promoter upstream of the first loxP site. Other features include drug selectable markers such as NPT (neomycin phosphotransferase) and HPT (hygromycin B phosphotransferase) to allow selection for the desired orientation, and a fluorescence marker, enhanced green fluorescence protein (EGFP). IRES – Internal Ribosome Entry Site, MCS – Multiple Cloning Site, PCMV – Cytomegalovirus promoter, pA – polyadenylation signal.