Bernard-Soulier syndrome is an uncommon bleeding disorder caused by a quantitative or qualitative defect in the platelet glycoprotein (GP)Ib/IX complex. The complex is composed of four subunits, GPIbalpha, GPIbbeta, GPIX, and GPV. Here we describe the molecular basis of a novel Bernard-Soulier syndrome variant in a patient in whom GPIbalpha and GPIX were undetectable on the platelet surface. DNA sequence analysis showed normal sequence for GPIbalpha, GPIX, and GPV. The GPIbbeta gene has been mapped to the 22q11.2 region of chromosome 22 which was deleted from one chromosome of this patient. There was a single nucleotide deletion within the codon for Ala 80 in GPIbbeta within the other allele. This mutation causes a translational frame shift that encodes for 86 altered amino acids and predicts a premature stop 15 amino acids short of the length of the wild-type protein. Transient coexpression of the mutant GPIbbeta in 293T cells with wild-type GPIbalpha and GPIX resulted in the surface expression of GPIbalpha, but the absence of GPIX. Moreover, when a plasmid encoding the wild-type GPIbbeta was transiently transfected into Chinese hamster ovary cells stably expressing GPalpha, which retain the capacity to reexpress GPIX, there was a significant increase in the surface expression of GPIX. In contrast, when the mutant GPIbbeta was transiently transfected into these cells, GPIX was not reexpressed on the plasma surface. Thus, a deletion of one copy of GPIbbeta and a single nucleotide deletion in the codon for Ala 80 within the remaining GPIbbeta allele causes the Bernard-Soulier phenotype through an interaction of GPIbbeta with GPIX resulting in the absence of GPIbalpha on the plasma membrane. The interaction of GPIbbeta with GPIX is essential for the functional expression of GPIbalpha.