The transmembrane glycoprotein Tumor endothelial marker 1 (TEM1) is highly expressed in tumor endothelial cells however it is barely detectable on normal endothelial cells. It is believed to play a role in tumor angiogenesis. It is expressed in stromal fibroblasts, metastatic hepatic lesions and during angiogenesis of corpus luteum formation and wound healing. TEM1 is being investigated as a potential target for cancer treatment.