RNAi screening strategies. (a) Gonczy et al.  injected dsRNA into the gonads of hermaphrodites (each injection targeted two genes) and scored their progeny for mutant phenotypes in two screens: a time-lapse DIC microscopy screen of meiosis and the first two embryonic cell divisions; and a dissecting-microscope screen for lethal or visible phenotypes. A total of 281 genes with phenotypes were identified; 130 of the 133 genes identified in the DIC screen were also lethal. A similar strategy, using a lethal screen followed by a DIC time-lapse screen, was followed by Piano et al. , who identified 81 genes with phenotypes in a screen of 350 cDNAs expressed in the ovary. (b) Fraser et al.  generated bacterial strains that produced dsRNA, fed these bacteria to worms, and scored progeny in a dissecting-microscope screen for lethal or visible phenotypes.