The ultimate goal of hemostasis is to stop bleeding by the conversion of soluble fibrinogen into insoluble fibrin to form a blood clot. Fibrinogen is a 340 kDa hexameric glycoprotein composed of two copies of three polypeptide chains: Aα, Bβ and γ, which are encoded by three different genes: FGA, FGB and FGG, respectively. Congenital fibrinogen disorders are caused by mutations in the three fibrinogen genes. Afibrinogenemia, where there is no detectable plasma fibrinogen; and dysfibrinogenemia, where the concentration of plasma fibrinogen is discordant with its functional activity, can lead to bleeding and thrombotic complications. In the first part of the thesis, our first aim was to study the pathophysiology of afibrinogenemia and In a subclass of fibrinogen molecules, referred to as Fibrinogen 420, each of the two Aα-chains is replaced by an AαE isoform. Since this fibrinogen subclass is preserved throughout the vertebrate kingdom, it is thought to have an important physiological function. In the second part of this thesis, we aimed to study the role of the AαE isoform in hemostasis and thrombosis using zebrafish as the model organism.