CD105, also known as Endoglin, is a disulfide-linked homodimeric type I transmembrane glycoprotein (1, 2). Two isoforms of CD105, a short form and a long form, differ by 33 amino acids (aa) in their cytoplasmic tails (3). The long form (180kD in size) is considered the predominant form of the protein although both isoforms have been detected by RT-PCR in endothelial cells, monocytic cell lines, and placenta (3, 4). CD105 serves as the regulatory component of the TGF-Beta receptor complex by associating with TGF-Beta receptor I and II (5, 6). These receptor complexes efficiently bind TGF-Beta1 and TGF-Beta3 with high affinity (5, 7). CD105 also binds other members of the TGF-Beta superfamily including activin-A, BMP-7 and BMP-2 (5). Functionally, CD105 plays an important role in angiogenesis (7, 8), hematopoiesis (9, 10) and cardiac development (9). CD105 was also shown to be a functional marker that defines long-term repopulating hematopoietic stem cells from the bone marrow