JAG1 (Jagged 1): JAG1 is a key ligand in the Notch signalling pathway, involved in regulating cell fate and tissue development. Mutations in JAG1 can cause Alagille syndrome, a disorder affecting the liver, heart, and other organs. Through its role in Notch signalling, JAG1 also contributes to processes such as vascular development, blood cell formation, and cancer progression.