A 3.5kb fragment of the elav promoter drives expression of 'SNTG4', an engineered receptor that can be used to monitor cell-cell interactions in vivo. The SNTG4 protein is a fusion of 1. a single-chain antibody domain (SCAD) that recognizes the extracellular domain of Mmus\Cd19, 2. N amino acid residues 1460-1767, encoding the NRR (Notch regulatory region, a fragment of the extracellular domain) and TMD (transmembrane domain), and 3. the GAL4(esn) driver. If SNTG4 is expressed in one cell type, and an appropriate ligand protein (containing the extracellular domain of Mmus\Cd19, the binding partner of SCAD) is expressed on a cell type that forms close cell-cell contacts with the SNTG4-expressing cell, then upon binding of SNTG4 and the Mmus\Cd19 fragment, components of the endogenous Notch/Delta signaling pathway cleave the N sequences in the NRR and TMD, releasing the GAL4(esn) driver, which can then enter the nucleus and activate transcription of a UAS-driven construct.