SummaryHeterozygozity for four novel missense mutations (W108C, W342R, E349K and L485S) and one novel 4 bp deletion (ACdelAAAG affecting codons 632-633) was identified in PROS1 of unrelated thrombosis prone Danish families with protein S type I or III deficiency. The 4 bp deletion results in a frameshift leading to replacement of the coding sequence for the 3 C-terminal amino acids by an abnormal extended sequence that codes for 9 amino acids. The E349K substitution was found in 7 families. Haplotype analysis using 7 microsatellite markers flanking PROS1 was consistent with a common founder for this mutation. The mutations reported here are most likely the cause of the protein S deficiency. Firstly, the four missense mutations cosegre-gate with the abnormal plasma protein S phenotype and lead to the loss of highly conserved amino acids. Secondly, computer analysis of structural models of protein S predicts that the substitutions could affect proper protein folding and/or stability. Analysis of platelet mRNA from subjects with the W108C, E349K, L485S mutation or the 4 bp deletion showed that mutated mRNA was expressed in significant amounts suggesting that mutated molecules are synthesized. Our results are compatible with defective protein folding/unstable molecules, impaired secretion and intracellular degradation of mutated protein, which appear to be the major molecular disease mechanisms for missense mutations and certain other mutations found in genetic disorders.