The mutation D442N(10372 G → A) in the active site of the human FXII serine protease detected in case A-148, a woman with three recurrent abortions and a CRM-positive phenotype. Her coagulation parameters were as follows: prothrombin time, 92%, partial thromboplastin time 27.6 seconds, 29.9 seconds; thrombin time, 17.8 seconds; fibrinogen (Clauss), 3.2 g/L; FII, 109%; FV, 97%; FVII, 89%; FX, 120%; FVIII, <120%; FIX, <120%; FXI, 111%; FXII:C, 63%; FXII:Ag, 100%; protein S (total), 135%; protein S (free), 130%; protein C, 110%; AT III, 107%. No antiphospholipid antibodies and no factor V Leiden mutation.39 (A) Schematic structure of the active site formed by the catalytic triad of Asp442, His393, and Ser544 (according to chymotrypsin; see Fersht31 ). (B) The replacement of Asp442 by Asn gives rise to a dysfunctional structure. (C) Reverse wild-type sequence. (D and E) Heterozgous form: reverse mutated and wild-type sequences are superimposed. The arrow indicates the mutation.