Fig. 1.

The siRNA pathway in Drosophila. The three core proteins of the RNAi machinery (Dicer-2, R2D2 and AGO2) associate with cofactors to sense RNAs and form the RNA-induced silencing complex (RISC). The siRNA pathway can be activated by the delivery of exogenous dsRNA in the cytosol or by viruses, but also by the transcription in the nucleus of natural antisense transcripts (NATs) or structured RNA. Virus-encoded suppressor molecules (in red triangles) inhibit the pathway at different steps (see the text for details)