The F13B gene encodes the B subunit of the coagulation factor XIII (FXIII). FXIII is the last zymogen to become activated in the blood coagulation cascade. It circulates in blood as a heterotetramer composed of two A- and two B-subunits that are non-covalently associated, designated as A2B2. The A subunits have enzymatic function. The B subunits, which are non-catalytic, serve a carrier function to stabilize the A-subunit in the aqueous environment of human plasma. FXIII is activated by cleavage of the peptide bond between Arg37 and Gly38 of the A-subunit by thrombin followed by subsequent dissociation of the B-subunit from the A-subunit in the presence of calcium ions, yielding the active FXIII enzyme (FXIIIa) composed of 2 activated A subunits. FXIIIa acts as a transglutaminase to catalyze the formation of gamma-glutamyl-epsilon-lysine cross-linking between fibrin molecules, thus rendering the fibrin clot chemically and mechanically more stable as well as resistant to fibrinolysis. Therefore, plasma FXIII plays an important role in wound healing and tissue repair, and it is essential to maintaining pregnancy.

Factor XIII deficiency is an inherited hemorrhagic disease characterized by a lifelong bleeding tendency and abnormal wound healing in affected patients and spontaneous abortion in female patients. This disease had been classified by the presence or absence of antigens into two categories: type I, characterized by the lack of both A- and B-subunits; and type II, characterized by the lack of A-subunit alone. Recently, it is proposed a new classification of factor XIII deficiency at DNA level: XIIIA (former type II deficiency) and XIIIB deficiency (former type I deficiency), and a possible combined deficiency of XIIIA and XIIIB.

The F13B gene is located on the long arm of chromosome 1 at 1q31-q32.1 and spans approximately 28 kb of genomic DNA with a coding sequence consisting of 12 exons interrupted by 11 introns. The 12 exons in the gene range in size from 64 to 222 base pairs, while the introns range in size from 87 to 9970 nucleotides and made up 92% of the gene. This gene is expressed in the liver encoding the B subunit of plasma FXIII. The B subunit is composed of 641 amino acids divided into 10 tandem repeats of about 60 amino acids each, which are called GPI structures, short consensus repeats (SCR) or Sushi domains. Each Sushi domain is encoded by a single exon. The function of these domains has yet to be fully established although they are thought to serve mainly as protein-binding module. The B-subunit has been shown to be highly polymorphic.