The Notch signaling pathway is an intercellular signaling mechanism that is essential for proper embryonic development. Members of the Notch gene family encode transmembrane receptors that are critical for various cell fate decisions. Jagged-1 (JAG1) is a cell surface protein which is one of several ligands that activate Notch and related receptors. JAG1 functions in an embryologically important signaling pathway and has also been shown to play a role in hematopoiesis. JAG1 inhibits proliferation of cd34+ macrophage progenitor cells and a JAG1 gene abnormality may be an aggravating factor in extrahepatic biliary atresia. Activation of NOTCH1 signaling by JAG1 induces monocyte-derived dendritic cell maturation in vitro. JAG1 interaction with NOTCH1 on tumor cells dramatically induces proliferation and inhibition of apoptosis in vitro. Either haploinsufficiency for wild-type JAG1 and/or dominant negative effects produced by mutated JAG1 are responsible for the Alagille syndrome.