As a complement to cell-based screening methods, it is also possible to screen for host genes that are required to combat pathogen infection using intact flies. Although these screens are more laborious than screens in Drosophila cell lines, or wholegenome RNAi screens in worms (i.e. Caenorhabditis elegans screens can be done on plates), screens using intact flies can be accomplished either by classic mutagenesis or by screening high quality collections of stable UAS-RNAi stocks. A great advantage of the latter approach is that one can use the GAL4-UAS expression system (Brand and Perrimon, 1993) to drive expression of UAS-RNAi constructs throughout the organism or in specific subsets of cells or stages of development (Fig. 2B ). For such experiments, a strain of flies carrying a transgene under the control of the yeast upstream activating sequence (UAS) is crossed to a strain of flies expressing the GAL4 transcription factor (which binds to the UAS sequence and activates transcription in a particular pattern, e.g. in the gut). The progeny then express the UAS transgene of interest in the pattern determined by the GAL4 'driver' stock, permitting expression of genes in specific cell types at specific stages of development. This level of control permits investigators to identify the cells or organs in which gene functions are required [e.g. epidermis, fat body (the main source of systemic AMPs, and an approximate model of the mammalian liver), hemocytes or gut].