siRNA levels in med18+and med20+deletion strains. (A) Representative Northern blot of siRNA in wild type and indicated mutants. Total RNA was run on a 17.5% polyacrylamide/7M urea gel, blotted and hybridized as described in Materials and Methods. Ethidium-bromide staining of the same RNA preparations was used as loading control. (B) Quantification of the blots (n = 4) *P <0.05; **P =5.2e to −12. The strains for this figure were: WT (FY498), med18Δ (MT42), med20Δ (MT26), and dcr1Δ (TP480).