The identification of cell surface markers expressed selectively by tumor vasculature is challenging. To get as close to the human disease as possible, investigators have isolated endothelial cells from fresh human tumor specimens and subjected them to RNA-based gene expression analysis. The data indicate that there are few proteins that distinguish tumor vasculature from normal vasculature and re-enforce the notion that the endothelium is a tissue specialized cell-type. Endosialin was identified as a cell surface tumor endothelial marker. The selective expression of endosialin by tumor vasculature and stroma has been confirmed. Although the function of endosialin remains to be elucidated, the expression pattern for this protein may be favorable for cancer therapy.