Tamoxifen-inducible endothelial Cre activity during angiogenesis revealed by activation of the R26R allele. Tumor blood vessels stained positive for LacZ: representative histologic sections from a day-14 LLC tumor (A, n = 5) and a day-12 B6RV2 lymphoma (B, n = 4) dissected from adult end-SCL-Cre-ER^T;R26R mice. The majority of LLC tumor endothelium costained for LacZ and CD31 (C-D). Arrows point to CD31⁺ vessels, which did not stain for LacZ (C). A representative CD31⁺ vessel is shown incorporating LacZ-positive endothelial cells (D inset). Lateral view of end-SCL-Cre-ER^T;R26R cornea 12 days after induction of corneal neovascularization (E). Newly formed blood vessels infiltrating the cornea stained positive for LacZ (whole-mount stain). (A-E) Two milligrams of tamoxifen were administered every 48 hours commencing one day before tumor cell implantation or corneal injury, respectively. Original magnifications × 20 (A-C); × 40 (D, D inset).