The pRL Vectors are wildtype Renilla luciferase (Rluc) control reporter vectors. The pRL Vectors, which provide constitutive expression of Renilla luciferase, can be used in combination with a firefly luciferase vector to cotransfect mammalian cells. Expression of Renilla luciferase provides an internal control value to which expression of the experimental firefly luciferase reporter gene may be normalized. The pRL Vectors contain the cDNA encoding Renilla luciferase (Rluc) cloned from the anthozoan coelenterate Renilla reniformis (sea pansy). Four different promoter configurations are available. The HSV-thymidine kinase promoter (pRL-TK) is relatively weak and may be particularly useful in providing neutral constitutive expression of the Renilla luciferase control reporter. The early SV40 enhancer/promoter region (pRL-SV40) and the CMV immediate early enhancer/promoter region (pRL-CMV) typically provide high-level transcription and, therefore, may be less suitable for co-reporter applications involving experimental vectors with robust regulatory elements. In general, we recommend validating the performance of specific co-reporter combinations in the desired target cells. In addition to the modified Rluc reporter gene, all pRL Vectors are isolated from a dam-/dcm- E. coli K host strain, allowing digestion with restriction enzymes that are sensitive to dam and dcm methylation.