Endoglin is a transmembrane glycoprotein 633 residues in length expressed at the surface of endothelial cells as a disulphide-linked homodimer; the specific cysteine residues involved in endoglin dimerization are unknown. Mutations in the coding region of the endoglin gene are responsible for hereditary haemorrhagic telangiectasia type 1 (HHT1), a dominantly inherited vascular disorder. Many of these mutations, if translated, would lead to truncated forms of the protein. It is therefore of interest to assess the protein expression of different truncated forms of endoglin. Infections in vitro or in vivo with recombinant vaccinia virus, as well as transient transfections with expression vectors, were used to express normal and truncated forms of endoglin. Truncated mutants could be classified into three different groups: (1) those that did not produce stable transcripts; (2) those that produced stable transcripts but did not secrete the protein; and (3) those that secreted a soluble dimeric protein. This is the first time that a recombinant truncated form of endoglin has been found to be expressed in a soluble form. Because a chimaeric construct encoding the N-terminal sequence of platelet/endothelial cell adhesion molecule (PECAM-1) antigen fused to residues Ile281-Ala658 of endoglin also yielded a dimeric surface protein, these results suggest that cysteine residues contained within the fragment Cys330-Cys412 are involved in disulphide bond formation. Infection with vaccinia recombinants encoding an HHT1 mutation did not affect the expression of the normal endoglin, and did not reveal an association of the recombinant soluble form with the transmembrane endoglin, supporting a haploinsufficiency model for HHT1.