The development of cancer is a cellular process that reflects and is partly driven by alterations in cell determination. Mutations in various genes responsible for cell determination have been identified as being oncogenic; however, little is known about the involvement of normal cell fate-determining mechanisms in the oncogenic process. The Notch pathway is an evolutionarily conserved, general cell-interaction mechanism that controls fundamental aspects of cell determination during the development of vertebrates and invertebrates. We have explored the roles of the recently cloned human Notch4 gene, which is a homologue of Drosophila Notch, in endometrial tissue, a cellular environment where cell fate changes take place and neoplastic conditions have been characterized. Northern blot analysis, dot blot analysis, and in situ hybridization were performed on samples of normal endometria at different phases of the menstrual cycle and endometrial cancers. Our results showed that: a) Both human Notch4 and translocation-associated Notch homologue-1 (TAN-1), another human homologue of Drosophila Notch, are expressed in the normal endometrium, mainly in the endometrial glandular cells. b) The level of expression of human Notch4, but not TAN-1, changes during the normal menstrual cycle; i.e., the level of Notch4 expression was significantly lower during the secretory phase than during the proliferative phase. c) Endometrial cancers express a significantly lower level of human Notch4 and a significantly higher level of TAN-1 than normal endometria. These results suggest that human Notch4 and/or TAN-1 are involved in the changes of the endometrium during the menstrual cycle, and also in the development of endometrial cancer.