Bioluminescence images of mice intravenously injected with 103 HeLa cells expressing Fluc (left) or Akaluc (right). Substrate administration was performed intraperitoneally. Images were acquired using a cooled CCD camera (1-min exposure time). The AkaLumine-HCl/Akaluc signals were statistically compared to D luciferin/Fluc signals. Data are presented as mean ± SEM of n = 3 mice.