Purity ~90% (SDS-PAGE). Endoglin (CD105) is a 90kD type I transmembrane glycoprotein of the zona pellucida (ZP) family of proteins (1-3). Endoglin and betaglycan/TbRIII are type III receptors for TGF beta superfamily ligands, sharing 71% aa identity with within the transmembrane (TM) and cytoplasmic domains. Endoglin is highly expressed on proliferating vascular endothelial cells, chondrocytes, and syncytiotrophoblasts of term placenta, with lower amounts on hematopoietic, mesenchymal and neural crest stem cells, activated monocytes, and lymphoid and myeloid leukemic cells (2-5). Human Endoglin cDNA encodes 658aa including a 25aa signal sequence, a 561aa extracellular domain (ECD) with an orphan domain and a two-part ZP domain, a TM domain and a 47aa cytoplasmic domain (1-3). An isoform with a 14aa cytoplasmic domain (S-endoglin) can oppose effects of long (L) Endoglin (6, 7). The human Endoglin ECD shares 65-72% aa identity with mouse, rat, bovine, porcine and canine Endoglin. Endogli