Bernard Soulier Syndrome (BSS) is a rare inherited bleeding disorder caused by a defect in the glycoprotein (GP)Ib/IX/V complex. A patient with a bleeding problem was diagnosed as having BSS based on the prolonged bleeding time, the absence of ristocetin induced platelet aggregations, thrombocytopenia and the presence of giant platelets. Analysis of the platelets of the propositus, a 39-year-old Belgian female, by flow cytometry revealed a decreased expression of the GPIb/IX polypeptides. Western blotting confirmed these results and showed moreover that there was a decreased disulfide bridge formation between GPIb alpha and GPIb beta. After sequence analysis of the GPIb alpha, GPIb beta and GPIX genes, only a mutation in the GPIX gene at position 1826 (A-->G) was identified, changing Asn45-->Ser. Restriction analysis with Fnu4H1 demonstrated that the patient was homozygous for this mutation. As this Asn45-->Ser mutation in the GPIX gene was already found in four unrelated families, i.e. in a British, Austrian, Swedish and Finnish one, the occurrence of this mutation in a Belgian patient supports the hypothesis of Koskela et al. (1999) that the Asn45Ser mutation in GPIX appears to be an ancient mutation shared by northern and central European populations. Our present observation of a decreased disulfide bridge formation between GPIb alpha and GPIb beta shows that GPIX is not only needed for the correct assembly of the complex but might also be needed for the disulfide bridge formation between GPIb alpha and GPIb beta.