Results of RNAi screening . RNAi screening identified genes that are (1) synthetic lethal with prdx-2 or (2) required for stress-induced, intestinal expression of a phase 2 detoxification transcriptional reporter gene (gcs-1p::gfp). First, 16,255 RNAi clones were screened in prdx-2 gcs-1p::gfp background, which contains increased, detectable `low' levels of intestinal GFP (indicated by arrows for representative animals and illustration) in addition to wild-type levels of constitutive pharyngeal GFP expression (indicated with `P') . Of the 951 RNAi clones that produced embryonic lethal (emb), lethal (let) or sterile (ste) phenotypes (and hence too few prdx-2 mutant progeny for gcs-1p::gfp to be scored), 237 had not previously been reported to affect growth or reproduction in screens of wild-type animals . Following re-screening of these 237 genes three times, 70 RNAi clones repeatedly produced emb, let and/or ste phenotypes (Additional file 1: Table S1). Of the 15,304 wells containing at least 40 viable progeny in the initial screen, 50 RNAi clones were scored that increased intestinal gcs-1p::gfp expression, such that more than 50% of animals in the well had a `high' level of gcs-1p::gfp expression throughout the intestine (see representative animal and illustration) (Additional file 1: Table S2). Another 355 RNAi clones reduced gcs-1p::gfp expression, such that more than 50% of animals in these wells had no detectable intestinal gcs-1p::gfp expression (`None'; see representative animal and illustration) (Additional file 1: Table S3). These 355 RNAi clones were re-screened three times and subjected to secondary screens: (i) 21 clones were discovered to reduce the intestinal expression of non-phase 2 gene F09E5.3::gfp; (ii) 24 clones reduced intestinal gst-4p::gfp expression (Additional file 1: Table S3); (iii) 90 clones reduced gcs-1p::gfp expression in prdx-2 mutants on each of four separate occasions (Additional file 1: Table S3); and (iv) 16 of these clones reduced arsenite-induced intestinal gcs-1p::gfp expression in wild-type worms (N2) in each of three trials but did not reduce F09E5.3::gfp expression (Table 1). GFP, green fluorescent protein; RNAi, RNA interference.