Protein jagged-1 I, also known as Jagged-1, JAGL1, HJ1, JAG1 and CD339, is a single-pass type I membrane protein. JAG1 contains one DSL domain and sixteen EGF-like domain. JAG1 acts as a ligand for multiple Notch receptors and is involved in the mediation of Notch signaling. JAG1 may participate in early and late stages of mammalian cardiovascular development, JAG1 inhibits myoblast differentiation and enhances fibroblast growth factor-induced angiogenesis. Defects in JAG1 are the cause of Alagille syndrome type 1, which is autosomal dominant multisystem disorder defined clinically by hepatic bile duct paucity and cholestasis in association with cardiac, skeletal, and ophthalmologic manifestations.