Due to its high specificity, RVV-X is a remarkablebiotechnologicaltool widely used in coagulation research and medical diagnostics.5,31,42 For example, the Stypven timeis determined by a clotting assay that measures the FX to FXa conversionby RVV-X and subsequent prothrombin activation that initiates clotformation.43−45 In particular, a long Stypven time is a diagnosticof FX deficiency, also called Stuart–Prower factor deficiency.44 Here, F10 is linked to complement factor I deficiency.