Promoterless firefly luciferase vectors are designed primarily to accept a putative promoter element for investigation of important regions controlling gene transcription. The promoterless vectors are available with three varieties of engineered firefly luciferase genes: luc2, luc2P or luc2CP. The luc2 gene is engineered to remove most cryptic transcription factor binding sites and improve mammalian expression through codon optimization. The luc2P and luc2CP and RapidResponse genes are luc2 genes appended with degradation sequences to influence the cellular half-life of the luc2 gene. The RapidResponse genes respond more rapidly to stimuli but at the expense of signal intensity. The luc2P (1-hour half-life) gene responds more rapidly than luc2 (3-hour half-life) with moderate signal intensity, and the luc2CP (0.4-hour half-life) responds more quickly with the lowest signal intensity. The promoterless pGL4.14[luc2/Hygro], pGL4.15[luc2P/Hygro] and pGL4.16[luc2CP/Hygro] Vectors are available with the selectable marker hygromycin.