Mutants of the DNA sequence coding for the protease (FSAP) which activates blood clotting factor VII and single-chain plasminogen activators, the mutants comprising a G/C base exchange at nucleotide position 1177 and/or a G/A base exchange at nucleotide position 1601, are described. The corresponding protease has a Glu/Gln exchange at amino acid position 393 and/or a Gly/Glu exchange at amino acid position 534. Diagnostic methods which are used for detecting FSAP in body fluids or tissue cells and also for identifying patients with genetic heterozygous or homozygous FSAP expression are also described. In addition, antibodies against FSAP and its mutants are disclosed and diagnostic methods which can be used to detect antibodies against FSAP and its mutants are specified.