 Exogenous dsRNA is detected and bound by an effector protein , known as RDE @-@ 4 in C. elegans and R2D2 in Drosophila , that stimulates dicer activity . This protein only binds long dsRNAs , but the mechanism producing this length specificity is unknown . This RNA @-@ binding protein then facilitates the transfer of cleaved siRNAs to the RISC complex . 
