Tumor endothelial marker (TEM) 1 was originally identified as a human embryonic fibroblast-specific antigen and was later determined to be endosialin, a single-pass transmembrane glycoprotein that has multiple extracellular domains, including three EGF-like domains, a sushi-like domain, and a C lectin-like domain. TEM proteins are significantly up-regulated during angiogenesis and neoangiogenesis that are crucial for the growth of solid tumors. While TEM1 is not required for angiogenesis during fetal development, postnatal growth or wound healing, it plays a role in tumor growth, invasion, and metastasis. Fibronectin and collagen types I and IV act as specific ligands of TEM1, leading to suggestions that these molecules may cause changes in the extracellular matrix, cell adhesion and migration during tumor invasion. At least two isoforms of TEM1 are known to exist; this antibody recognizes only the larger isoform.