Fig 1. Reporter gene imaging. A, Schematic diagram of the principle of reporter gene imaging by using the enzyme firefly luciferase. Once the cell is transduced with a viral vector containing the imaging gene cassette, a promoter of choice drives the transcription of the imaging reporter gene (Fluc). If the promoter leads to transcription of Fluc, then translation of the imaging reporter gene mRNA leads to a protein product (the enzyme firefly luciferase) that can interact with the imaging reporter probe (D-Luciferin). This interaction is a chemiluminescent reaction that catalyzes the transformation of the substrate D-Luciferin into oxyluciferin in a process dependent on ATP, Mg⁺⁺, and O₂, leading to the emission of light, which can be detected by using low-light sensing instruments. Other gene/substrate combinations may be used as well (eg, hRluc and its substrate CL, see text). B, Bioluminescence neuroimaging in mice. Balb/c mice with 10⁵ intracranially injected N₂a cells transfected 24 hours previously with CMV-hRluc (right) and CMV-Fluc (left). Intracranial injections were performed immediately before substrate administration. The mice received intraperitoneal injections of the substrates D-Luciferin (1.5 mg) or CL (5µg) respectively. The charge-coupled device camera images were taken approximately 5–7 minutes post-substrate injections. Maximum signal intensity detected in photons per second per square centimeter per steradian is the following: Fluc, 2.1 x 10⁶; hRluc, 9.2 x 10⁴. CMV indicates cytomegalovirus; CMV-FL, adenoviral vector containing an imaging cassette with the CMV promoter driving the transcription of firefly luciferase gene.