LacZ staining of tissue sections. Cryosections were collected from heart, lung, kidney, and skeletal muscle (Sk mu) harvested from 6- to 8-week-old F2 male Hprt-targeted mice carrying wild-type, −39 ETSm, or EGRm VEGFR1 transgenes and stained with X-Gal. Serial sections were processed for CD31 immunohistochemistry. Images were obtained using a 20× objective. Bar, 100μM. Lac-Z-stained sections were counterstained with eosin. Slides were analyzed under a Zeiss Axio Imager upright microscope, and photomicrographs were collected using a Zeiss Axiocam MRc camera and Axiovision 4.6.3 image acquisition software.