Tissue susceptibility to RNAi by feeding. Worms with a transgenic GFP reporter gene expressed in all somatic tissues (egl-27::gfp) were fed (a) non-dsRNA expressing bacteria or (b-e) bacteria expressing gfp dsRNA for (b) 24 h, (c) 48 h, (d) 72 h, or (e) 96 h. After being fed for 24 h, GFP expression was markedly reduced (b) compared to similarly treated unfed worms (a). (c) Of worms fed for 48 h, none had any visible non-neural GFP expression, and 20/22 (91%) had reduced levels of neural GFP. (d) After 72 h, 93% had no non-neural somatic GFP expression (the remaining 7% had very weak expression), and 28/30 (93%) had reduced neural GFP. (e) Finally, after being fed for 96 h, no worms had any non-neural GFP expression, and 26/27 (96%) also had reduced levels of neural GFP compared with similarly treated unfed worms. The head of each worm (from the nose to the posterior pharynx), which contains the majority of the neurons in the animal, is indicated with a bracket.