F13B encodes the B chain of coagulation factor XIII, a non-catalytic subunit that stabilizes the catalytically active A subunits and regulates transglutaminase formation by thrombin 1. The tetrameric FXIII-A₂B₂ zymogen is converted to active FXIIIa, which cross-links fibrin chains and alpha-2 plasmin inhibitor to mechanically stabilize fibrin clots and protect them from fibrinolysis 1. While F13A deficiency causes severe hemorrhagic diathesis with characteristic delayed umbilical stump bleeding, impaired wound healing, and spontaneous abortion, F13B subunit deficiency results in milder bleeding symptoms 1. Beyond hemostasis, F13B variants associate with age-related macular degeneration at the CFH-to-F13B locus, where homozygous risk genotypes correlate with elevated terminal complement complex levels in ocular tissue 2. Recent evidence suggests F13B functions in cancer progression; in hepatocellular carcinoma, reduced F13B expression promotes cell invasion and migration, while F13B overexpression inhibits angiogenesis through the HIF-1α/VEGF pathway 3. F13B also appears involved in pregnancy maintenance, with specific SNP variants influencing recurrent pregnancy loss risk 4. Diagnosis of F13 deficiency requires quantitative activity assays and antigen measurement, as mutations are heterogeneous without hot-spot regions 1.