Familial hypobetalipoproteinemia is caused by mutations in the apolipoprotein (apo) B gene. We identified a 57-year-old woman whose plasma total cholesterol and apoB levels were 2.17 mmol/L and 0.03 g/L, respectively. Separation of plasma lipoproteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the absence of apoB-100 and the presence of a faster-migrating form of apoB with an apparent Mr of 195 kDa. Direct sequencing of a polymerase chain reaction-amplified fragment of the patient's apoB gene DNA revealed a single C-->T transition at nucleotide 5472 that converts glutamine 1755 (CAA) to a stop codon (TAA). We predict this novel nonsense mutation of the apoB gene to produce a truncated protein that contains 1754 amino-terminal amino acid residues of apoB-100. We designated this mutant form of apoB apoB-38.7 by following the centile nomenclature of the apoB species. The same mutation was found in both of her children. The proband revealed clinical findings of retinitis pigmentosa, acanthocytosis, and loss of deep tendon reflexes that are characteristic of severe hypobetalipoproteinemia. In addition, the proband had type II diabetes mellitus with nephropathy, anemia, cholelithiasis, hepatic hemangioma, bronchiectasis, and extensive calcification of major arteries including, the celiac, splenic, and renal. In summary, we have found a novel truncated apoB, apoB-38.7, in a patient with an unusual presentation of hypobetalipoproteinemia that includes diabetes mellitus and extensive arterial calcification.