Involved in the Notch signaling pathway as Notch ligand (PubMed:11134954). Activates NOTCH1 and NOTCH4. Involved in angiogenesis; negatively regulates endothelial cell proliferation and migration and angiogenic sprouting (By similarity). Essential for retinal progenitor proliferation. Required for suppressing rod fates in late retinal progenitors as well as for proper generation of other retinal cell types (PubMed:22323600). During spinal cord neurogenesis, inhibits V2a interneuron fate (By similarity). Interacts with NOTCH4. Interacts (via N-terminal DSL and MNNL domains) with NOTCH1 (via EGF-like domains). Expressed in vascular endothelium. Expressed in retina at least during embryogenesis. At 14.5, expressed within the retina outer neuroblastic layer. Conditional knockdowns in retinal progenitors have thinner retinas with occasional regions abnormally organized into rosette-like structures in the outer nuclear layer and an optic nerve with a reduced diameter.