This invention relates to the diagnosis of congenital defects in the anticoagulant protein C system. Methods that are disclosed are based on the detection of mutations at the cleavage sites of coagulation factors that are under control of activated protein C (APC). Diagnostic tests include analysis of the APC-cleavage sites of factor V and factor VIII, by using specific primers to amplify selectively from RNA, cDNA derived from RNA or chromosomal DNA, parts of factor V and factor VIII that contain cleavage sites for APC. Methods that monitor the presence of mutations at the cleavage sites for APC and their utility in the diagnosis of thrombo-embolic disease are disclosed. The invention further discloses methods for correcting the defects detected according to the invention, as well as novel therapeutic agents which can be used in the treatment of bleeding disorders, which agents are based on the 'defective' Factor V and Factor VIII proteins leading to the thrombotic disorders described hereinabove.