Missense mutations, three of them novel (Gly47-->Cys, Arg178-->Pro, Ala259-->Thr), were found in the protein C genes of four patients with inherited protein C deficiency associated with venous thrombosis. Comparison with the phenotypic effects of mutations in the analogous residues of factor IX and the use of a molecular model of protein C provided explanations as to how these lesions might alter either the structure, function or stability of the protein C molecules encoded.