The mammalian Dicer is essential for miRNA production but was shown to be generally inefficient in supporting RNAi activity. The evolutionary specialization on miRNA processing demonstrates the importance of a tight endogenous gene regulation, while the lost RNAi activity was complemented by an interferon-based antiviral defense. Interestingly, Mus musculus (mouse) presents a unique case among mammals by expressing two Dicer isoforms: the full-length somatic Dicer (mDicer) and a truncated oocyte-specific variant (mDicerO). Unlike mDicer, mDicerO selects for a broader range of substrates and provides strong RNAi activity, which is essential for mouse oocyte development. However, the mechanisms by which the N-terminal truncation in mDicer reactivates RNAi and enhances long dsRNA processing in mDicerO remain unclear. In this PhD project, we will combine the biochemical and structural approaches (namely cryo-EM) to characterise the mechanism behind the RNAi reactivation.