Luciferase activity in transgenic mouse colonies. Luciferase activity in organ homogenates from two transgenic colonies (32 [n = 5] and 12 [n = 5]) derived from independent founders is presented. The mean and standard error of luciferase activity (RLUs per μg of protein) are shown. Maximal luciferase activity was observed in bone marrow and spleen, both hematopoietic organs in mice and both containing murine megakaryocytes. Luciferase activity was also detected in blood, but is not apparent in this figure owing to the small contribution of platelet protein to the total protein found in whole blood (see Results). Consistent, albeit low levels, of luciferase activity were observed in lung, heart, and aorta even after extensive perfusion of the mouse. The relevance of low levels of gene activity in these organs is discussed.