A 24-year-old male presented with progressive gait disturbance and was diagnosed with hereditary spastic paraplegia. His brother and possibly one uncle also had the condition. Routine biochemical testing found that the patient had unusually low plasma concentrations of low density lipoprotein (LDL) cholesterol and apolipoprotein (apo) B, the hallmark of familial hypobetalipoproteinemia. DNA sequencing showed that he, along with other family members (n=5; mean LDL cholesterol 0.8 mmol/L, apoB 0.31 g/L), were heterozygous for a single nucleotide deletion in exon 26 of the APOB gene. This mutation is predicted to form a truncated apoB species of 3545 amino acids, which we have designated apoB-78.2. Crown Copyright (c) 2008 Published by Elsevier B.V. All rights reserved.