Levels of doxycycline (Dox) in the serum of pGTTMIK-treated mice. Levels of Dox in the serum from mice that were administered sweetened water only (white bars) or Dox (200 μg/ml) (black bars) were determined using pGTLMIK transiently transfected 293T cells. The experiment was performed as decribed in Materials and methods. Dox concentration in the blood was determined from a standard curve of luciferase activity obtained with serum spiked with known Dox concentrations. Mean values are shown, and vertical lines represent the standard error of the mean.