Using the Gal4/UAS-system, activation of the salivary glands can be achieved by genetic means. Proteins of the salivary gland secretion family are believed to be restricted to the salivary glands. Crossing the sgs3::gal4 line with a UAS::gfp responder line revealed that expression is indeed restricted to the salivary glands (A). Based on the UAS/Gal4 system for ectopic activation, the IMD-pathway, which is instrumental in epithelial immunity, can be activated by targeted overexpression of peptidoglycan recognition receptors. The membrane bound PGRP-LC activates in a cell-autonomous way, whereas the soluble PGRP-LE activates in an organ-wide, systemic way (B). If the pgrp-le gene is expressed in the salivary glands only, the antimicrobial peptide gene drosomycin (here used as a reporter for immune system activity and visualized by drosP::gfp) is expressed in defined parts of the intestine only (but not on the salivary glands) (C).