Hemizygous transgenic mice are viable, fertile, normal in size, and do not display any gross physical or behavioral abnormalities. In the presence of tetracycline or a tetracycline analog such as doxycycline, selective expression of beta-galactosidase is observed in the nuclei of vascular endothelium in a wide variety of tissues: aorta, heart, brain, lung, kidney, liver, spleen, uterus, prostate, stomach, skeletal muscle, large and small intestine. Expression is observed as early as embryonic day 9.5. In the absence of tetracycline, some beta-galactosidase expression occurs in the smaller branches of the aorta. A trend towards diminished beta-galactosidase expression when the alleles are crossed onto a C57BL/6 background has been reported.