Publisher Summary CD105 is present on endothelial cells. It is absent from most normal B and T cells, but is present on some leukaemic cells of B lymphoid and myeloid origin and on a subset of bone marrow cells. The CD105 expression can be induced on the surface of in vitro activated macrophages and macrophage cell lines. CD105 is present at the cell surface as a disulfide-linked homodimer. O-glycosylation of CD105 has been demonstrated by digestion with O-glycanase. The O-glycosylation is likely to be on the membrane-proximal region that contains a high proportion of Thr, Ser, and Pro residues. The N-terminus has been established by protein sequencing. The sequence shows some patches of similarity with another receptor for TGFβ, namely, TGFβ receptor type III (β-glycan). The highest similarity is between the cytoplasmic domains at 70%, with an overall sequence identity of 30%. CD105 is one of several receptors for the various isoforms of TGFβ, which in turn is one of a family of proteins involved in regulation of cell differentiation, migration of cells, and control of the immune response. The level of CD105 increases in response to TGFβ.