Human RNase L inhibitor (RLI) was first identified as a component of the antiviral 2-5A/RNase L system, which could also play a more general role in regulation of RNA turnover. In contrast to RNase L, RLI proteins are highly conserved from archea to eukaryotes and are essential for viability in several organisms. This implies that there are other essential functions of RLI besides the RNase L pathway. Further studies revealed more diverse roles of the RLI proteins, although their general function is still not understood. The Arabidopsis homologue of RLI, AtRLI2, was identified as an endogenous suppressor of RNAi. One aim of this project is to study if human RLI has a similar role in RNAi suppression and to address the mechanism of the suppression on a cultured cell model. The second aim of the project is to characterize mitochondrial function of RLI, since a fraction of human RLI is localized in mitochondria. General phenotype of cells with downregulated RLI will be also characterized and more studies will be performed in order to understand if RLI function is essential for cell cycle progression and cell proliferation. The results of this study are expected to provide a new knowledge about RLI functions, which is still very limited.