Rigidity of recombinant BβArg448 and BβLys448 fibrin clots assessed by magnetic tweezers. Shown are G′ values in different time points (up to 45 minutes) of clots prepared from recombinant Arg448 and Lys448 variants (1.3 μM fibrinogen, 2.5 mmol CaCl2, and 0.5 U/mL thrombin) in the absence (A) and presence (B) of FXIII at concentrations of 0.068 μM. Error bars represent SD.