During development, the Notch signaling pathway is essential for the appropriate differentiation of many cell types in organisms across the phylogenetic scale, including humans. Notch signaling is also implicated in human diseases, including a leukemia and two hereditary syndromes known as Alagille and CADASIL. To generate tools for pursuing the role of the Notch pathway in human disease and development, we have cloned and analyzed the expression of three human homologues of the Notch ligands Delta and Serrate, human Jagged1 (HJ1), human Jagged2 (HJ2), and human Delta1 (H-Delta-1), and determined their chromosomal localizations. We have also raised antibodies to HJ1, and used these antibodies in conjunction with in situ hybridization to examine the expression of these ligands in normal and cancerous cervical tissue. We find that, as reported previously for Notch, the ligands are up-regulated in certain neoplastic tissues. This observation is consistent with the notion that Notch signaling is an important element in these pathogenic conditions, raising the possibility that modulation of Notch activity could be used to influence the fate of the cells and offering a conceivable therapeutic avenue.