Several homologs of the Drosophila Notch receptor and its ligands, Delta/Serrate, have been cloned in man. Three human disorders including a neoplasia (a T-cell acute lymphoblastic leukemia/lymphoma), a late onset neurological disease (CADASIL) and a developmental disorder (the Alagille syndrome) are associated with mutations in, respectively, the Notch1, Notch3 and Jagged1 genes, pointing out the broad spectrum of Notch activity in humans. We report herein on what has been learned on the role of these human Notch genes and the mechanisms leading from mutations in those genes to the observed phenotypes.