Endoglin (CD105) is a 90 kDa type I transmembrane glycoprotein of the zona pellucida (ZP) family of proteins (1‑3). Endoglin and betaglycan/T beta RIII are type III receptors for TGF-beta superfamily ligands, sharing 71% aa identity with 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). Rat Endoglin cDNA encodes 650 amino acids (aa) including a 25 aa signal sequence, a 553 aa extracellular domain (ECD) with an orphan domain and a two-part ZP domain, a TM domain and a 51 aa cytoplasmic domain (1‑3). In human and mouse, an isoform with a 14 aa cytoplasmic domain (S‑endoglin) can oppose effects of long (L) Endoglin (6). In rat, a potential isoform with a 100 aa cytoplasmic tail (49 aa inserted after aa 610) diverges at the same aa as S‑endoglin (7). The rat Endoglin ECD shares 84%, 70%, 68%, 64%, and 62% aa identity with mouse, human, canine, porcine, and bovine Endoglin, respectively. Endoglin homodimers interact with TGF-beta 1 and TGF-beta 3 (but not TGF-beta 2), but only after binding T beta RII (8). Similarly, they interact with activin-A and BMP-7 via activin type IIA or B receptors, and with BMP-2 via BMPR-1A/ALK-3 or BMPR-1B/ALK-6 (9). BMP-9, however, is reported to bind Endoglin directly (10). Endoglin modifies ligand-induced signaling in multiple ways. For example, expression of Endoglin can inhibit TGF-beta 1 signals but enhance BMP7 signals in the same myoblast cell line (11). In endothelial cells, Endoglin inhibits T beta RI/ALK-5, but enhances ALK‑1‑mediated activation (12). Deletion of mouse Endoglin causes lethal vascular and cardiovascular defects, and human Endoglin haploinsufficiency can cause the vascular disorder, hereditary hemorrhagic telangiectasia type I (13, 14). These abnormalities confirm the essential function of Endoglin in differentiation of smooth muscle, angiogenesis, and neovascularization (2‑4, 12‑14). In preeclampsia of pregnancy, high levels of proteolytically generated soluble Endoglin and VEGF R1 (sFLT1), along with low placental growth factor (PlGF), are pathogenic due to antiangiogenic activity (15).