 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.The mechanism producing this length specificity is unknown and this protein only binds long dsRNAs.In "C. elegans" this initiation response is amplified through the synthesis of a population of 'secondary' siRNAs during which the dicer-produced initiating or 'primary' siRNAs are used as templates.