In this report we show that endothelial Jagged1 acts as a pro-angiogenic ligand in a tumor setting, after having recently demonstrated this effect in a regenerative setting [11], where Jagged1 antagonizes Dll4 regulation of endothelial branching, by its ability to block Dll4/Notch1 activation and thus by positively regulating Vegfr-2 transcription. The gene discussed is JAG1; the disease is neoplasm.