As mentioned above, the RNAi pathway plays a key role in defending against viral infection. Genome-wide and targeted RNAi screens have contributed to the elucidation of this pathway (Galiana-Arnoux et al., 2006; Nayak et al., 2010; Otsuka et al., 2007; Sabin et al., 2009; van Rij et al., 2006; Wang et al., 2006) (for a review, see Sabin et al., 2010) and the importance of the systemic spread of an RNAi activating signal (probably some large viral doublestranded RNA) for stimulating RNAi-dependent immunity throughout the organism (Saleh et al., 2009) . Interestingly, siRNAs do not spread from cell to cell in Drosophila (Roignant et al., 2003) , in contrast to the mechanism by which RNAi molecules are directly distributed in plants (Palauqui et al., 1997; Winston et al., 2002) and nematodes (Fire et al., 1998; Voinnet et al., 1998) to mediate systemic immunity.