Recent progress in molecular biology enabled the elucidation of the nucleotide sequences of the genes for antithrombin III (AT III), protein C (PROC) and protein S (PROS). Furthermore numerous mutations were identified causing genetic defects of the important inhibitors of blood coagulation. As the genes for AT III (13.8 kb) and PROC (11.2 kb) are small and easy to analyze a great number of molecular defects already are described in extensive databases (50, 73): 79 different mutations for AT III and 160 for PROC are included. The identification of mutations leading to AT III and PROC deficiency has given important information on the structure-function relationships of the proteins. In case of protein C deficiency the clinical relevance of DNA analyses is most important because the diagnosis at the protein level is often uncertain. The gene for PROS is not so easy to analyze like the other two genes. The PROS gene is large and also a PROS pseudogene exists. Although a number of mutations have been identified, there has not been published a database until now. The clinical relevance to identify gene defects in PROS deficiency is as important as for PROC deficiency. Presumably the elucidation of PROS gene defects will advance in the near future.