Constitutive and regulated expression of luciferase in vivo. Plasmid DNA (30 μg) pcLuc+ (a) or pGTLMIK (b) was delivered by intramuscular injection with electroporation on day 0. Mice were imaged using the IVIS system on days 4, 15, 32, 71, and 168 (for the pcLuc+-treated mouse) after injection of luciferin substrate and anaesthetisation. Light emission (photons per steradian per square centimetre) was measured from the right leg of DNA-injected mice. For pGTLMIK-treated mice, luciferase expression (black bars) is compared with the left leg (white bars) and with a control (C) region of the same size on the abdomen (grey bar). Mice received doxycycline (Dox) (200 μg/ml) in sweetened drinking water for 15 days after DNA delivery. Dox was then removed until day 67, when the same concentration Dox drink was again supplied. Values for pcLuc+ are obtained from a single mouse, and for pGTLMIK the values are the mean from three mice; vertical lines represent the standard error of the mean. Images obtained from the three pGTLMIK-injected mice on days 15, 32, and 71 are shown on the right. These results confirm that luciferase expression from pGTLMIK can be very efficiently switched on (days 15 and 71) and off (day 32) by addition or removal of Dox from the diet.