In situ hybridization for murine tumor endothelial marker 1 (mTEM1) demonstrates specific mRNA expression (red staining) in endothelial cells lining the blood vessels infiltrating an implanted B16 melanoma. Five full length human cell surface TEMs were cloned and characterized along with their mouse counterparts. In situ hybridization analyses revealed a subset of these genes to be highly expressed in tumor endothelial cells as well as in the endothelium of developing embryos. These genes should prove to be important, accessible markers of tumor angiogenesis. For details, see the article by Carson-Walter et al. on p. 6649 in this issue.