F13A1 and systemic lupus erythematosus: Thus, the factor XIIIa-catalyzed γ-γ-dimerization of fibrin was enhanced in the clots obtained from the active SLE patients compared to those with inactive SLE, suggesting that the overall covalent fibrin stabilization, including γ- and α-chain crosslinking, may contribute the differences in the viscoelastic properties of fibrin that correlate with the severity of SLE.