Reporter vectors containing reporter genes are usually used to assess the transfection efficiency of a target gene into a cell, as well as the protein expression of the target gene. When the promoter regions are cloned upstream of the reporter gene and enhancer elements are cloned upstream or downstream of the gene, the reporter vector allows functional identification and characterization of promoters and enhancer elements that regulate cell, tissue, and developmental definitions, since the expression of the reporter is correlated with the transcriptional activity of the reporter gene. Reporter genes can also be engineered into viral vectors to track the type of cells the virus infects, the timing and duration of expression for viral genes, and viral latency. In addition, reporter vectors can be used to monitor other processes such as recombination events, gene targeting, RNA processing, protein secretion pathways, and signal transduction pathways in cells. In a transfection experiment, the use of control is vital for monitoring transfection efficiency and optimizing transfection method. Co-transfection with reporter vector is a strategy that can be used to assess transfection efficiency by expressing specific reporter proteins. The reporter gene that acts as a control is driven by a strong, constitutive promoter and is transfected with tested vectors. The regulatory sequence being tested is linked to the other reporter gene so that the relative activities of the two reporter genes can be determined individually. Alternatively, two reporter genes can be constructed on the same plasmid, which is called as dual-reporter vectors. A classical dual-reporter system consists of two different promoter elements, each controlling the transcription of different reporter genes.