Haemophilia A is one of the most common inherited bleeding disorders. Bleeding phenotype is caused by mutations within factor VIII gene, which result in deficient activity of the factor VIII in plasma. Various genetic defects have been described to affect factor VIII transcription, translation or its amino acid composition. Utilisation of 3D-model of the factor VIII helped to identify amino acid substitutions which adversely affect the protein folding and those which disturb various factor VIII interactions. Genetic defects have been stratified with respect to the relative risk of factor VIII inhibitor formation and a mechanism by which the antibody inhibits its function has been described. The knowledge has been utilised in current diagnostic practice and developement of future therapeutic strategies.

        Key words: haemophilia A, factor VIII, factor VIIIa, factor VIII gene, inhibitor, mutation, von Willebrand factor, factor IXa, tenase.