Glanzmann's thrombasthenia (GT) is the result of the absence or of an altered and dysfunctional expression on the platelet membrane of the fibrinogen receptor (glycoprotein [GP] IIb/IIIa complex). Various molecular genetic mechanisms have been found to be responsible for this inherited disease. In a patient with a severe type of GT, we have found a splice variant in the GP IIIa gene that leads to premature chain termination. Immunoprecipitation experiments, using monoclonal antibodies specific for GP IIb/IIIa, showed that GP IIb/IIIa was not detectable on the platelet membrane. Amplification of reversely transcribed platelet GP IIIa mRNA by the polymerase chain reaction and subsequent sequence analysis showed a 86-bp deletion, which corresponds to exon i of the GP IIIa gene. This deletion results in a shift of the reading frame leading to eight altered amino acids followed by a premature termination codon. Analysis of the corresponding genomic DNA fragments showed three mutations in the exon i-intron i boundary region of the GP IIIa gene. One of these mutations is a G-->T transition that eliminates the GT splice donor site in the wild type. This base pair change creates a restriction site for the enzyme Mse I. Allele-specific restriction enzyme analysis (ASRA) with Mse I of amplified genomic DNA of the parents and the proposita showed that both parents (who are first cousins) are heterozygous, whereas the proposita is homozygous for the G-->T substitution.