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Model of PolIV dependency of siRNA-generating loci. PolIV-independent epigenetic marks (blue stars) recruit PolIVa (pink ovals) to generate siRNAs. In the presence of PolIVb, additional epigenetic marks (yellow triangles) can reinforce recruitment of PolIVa. Additionally, RDR2-mediated amplification of siRNAs can occur when direct repeats are present in the transcript, as in LTR retroelement RNAs.
