A genetic condition caused by pathogenic variants in the NOTCH2 gene upstream of those implicated with Hajdu-Cheney syndrome. The mechanism of pathogenicity for Alagille syndrome appears to be haploinsufficiency. Key features include bile duct paucity, cholestasis, congenital heart defects, butterfly vertebrae, posterior embryotoxon, and distinctive facial characteristics. Renal abnormalities may also be present.