ENDOGLIN codes for a homodimeric membrane glycoprotein that interacts with receptors for members of the TGF-beta superfamily and is the gene mutated in the autosomal dominant vascular disorder hereditary hemorrhagic telangiectasia type 1 (HHT1). We recently demonstrated that functional endoglin was expressed at half levels on human umbilical vein endothelial cells (HUVECs) and peripheral blood activated monocytes from HHT1 patients. Two types of mutant protein were previously analyzed, the product of an exon 3 skip which was expressed as a transient intracellular species and prematurely truncated proteins that were undetectable in patient samples. Here we report the analysis of four proteins resulting from point mutations, with missense codons G52V and C53R in exon 2, W149C in exon 4 and L221P in exon 5. Metabolic labeling of activated monocytes from confirmed, clinically affected patients revealed reduced expression of fully processed normal endoglin in all cases. Pulse-chase analysis with HUVECs from a newborn with the C53R substitution indicated that mutant endoglin remained intracellular as a precursor form and did not impair processing of the normal protein. Biotinylation of cell surface proteins, metabolic labeling and pulse-chase analysis revealed that none of the engineered missense mutants was significantly expressed at the surface of COS-1 transfectants. Thus, these four HHT1 missense mutations lead to transient intracellular species which cannot interfere with normal endoglin function. These data suggest that haploinsufficiency, leading to reduced levels of one of the major surface glyco-proteins of vascular endothelium, is the predominant mechanism underlying the HHT1 phenotype.