We investigated two unrelated patients with Bernard-Soulier syndrome (BSS) by performing molecular and genetic analysis. A flow cytometric and immunoblotting analysis showed GP Ib alpha to be absent from the platelet membrane of both patients. Other glycoproteins that formed GP Ib/IX/V complex were present on the platelets, but in decreased amounts. Therefore, GP Ib alpha gene from both cases was sequenced after PCR amplification and subcloning. We identified a homozygous mutation of a dinucleotide deletion within the TGTG repeat at cDNA number 972 to 975 in GP Ib alpha gene from Case 1. In Case 2, compound heterozygosity was demonstrated in GP Ib alpha gene; an insertion of a single base (T) at cDNA number 1,418 in one allele, and a deletion of a single base (A) within the 7-adenine repeat at cDNA number 1,438 to 1,444 in another allele. The three new mutations in both patients appeared to cause a frameshift, which created a new termination codon shortly thereafter, and thus lead to a GP Ib alpha deficiency on the platelet membrane. Truncated mutant proteins could be detected in the plasma and platelets of Case 2, but not of Case 1. According to these findings, it is thus supposed that the properties and conformation of additional COOH-terminal peptides, which were supposedly synthesized as results of the mutations, may have an important role on the processing of mutant GP Ib alpha in megakaryocytes and platelets.