CD339, also known as Jagged 1, is a 135 kD type 1 transmembrane protein, with one DSL (Delta, Serrate, LagZ) domain, fifteen EGF-like domains, and one cysteine-rich region. Jagged 1 is expressed by many cell types including macrophages, subsets of dendritic cells, astrocytes, stromal, and epithelial cells. CD339 regulates cell fate decisions in various tissues, and has an important role in neurogenesis and development of the cardiovascular system. Studies suggest Jagged 1 is involved in Th2 polarization and Treg induction.