Previously, we characterized mouse and human cDNA clones encoding a new transmembrane EGF-like protein. The predicted protein from this cDNA (termed delta-like or dlk) is highly homologous to invertebrate homeotic transmembrane proteins, including Notch and Delta of Drosophila, also containing EGF-like repeats at their extracellular domains which intervene in protein-protein interactions between cells. These interactions generate signal transduction events which are involved in differentiation decisions. dlk is highly expressed in tumors of neuroendocrine origin. Our interest is to understand the function of dlk in those tumors, as well as in normal tissues. A second project initiated in our laboratory, refers to the study of the function of TAN-1 gene, a human homolog of the Drosophila Notch, in the immune system. Notch is a extensively studied gene in Drosophila, which intervene in differentiation decisions during the fly's development. The function of a highly conserved and highly expressed homolog of this gene in the immune system of adult mammals is unknown, but some evidence indicates that it may have a role in the origin of T-ALL. Our objectives for the year were to continue with the functional studies on the dlk, and to express dlk in bacteria in order to obtain antibodies against the protein. A partial dlk protein has been expressed in E. coli and purified. Production of rabbit antisera is ongoing. RNAse protection assays have been performed with hundreds of cell lines and fresh tumors, confirming the very selective expression pattern of dlk in neuroendocrine tissues, as well as a correlation between the differentiation state and dlk expression. The application of the differential PCR technique has allowed the identification of a gene upregulated in cells transfected with dlk, suggesting that dlk may intervene in events related to gene modulation. Genomic TAN-1 clones have been obtained. Cloning of a full- length TAN-1 cDNA is almost complete. Fusion proteins of the intracellular region of TAN-1, suspected to be involved in protein-protein interactions, have also been produced. Antisera production, as well as protein-protein interaction studies will be performed in the future.