One of the great recent technical advances in the field of Drosophila cell biology has been the development of efficient whole genome RNAi screens to identify genes required for specific cellular processes (Mohr et al., 2010; Perrimon and Mathey-Prevot, 2007; Perrimon et al., 2010) . In such assays, Drosophila cell lines such as hemocyte-derived S2 cells or Kc cells (which can be induced by hormone treatment to differentiate into neurons) are grown in 384well plates and treated with a library of double-stranded RNAs that have been designed for highly selective RNAi-mediated knockdown of each of the predicted Drosophila coding messenger RNAs (mRNAs). These cells are then assayed for performance of a cellular process such as cell viability, cell shape changes or bacterial uptake by phagocytosis. By screening such libraries in replicate and then re-screening RNAi candidates that test positive for a specific effect, it is possible to approximate genome-wide coverage of all genes required in these cells for a given process [for an excellent, comprehensive review of such RNAi screens, see Cherry (Cherry, 2008) ]. Such screens have been used to identify many host response factors that are crucial during infection by bacteria, fungi and viruses.