In somatic cells, the nucleolus is composed from three basic components and is primarily the site of ribosome biogenesis. In fully-grown mammalian oocytes a corresponding structure cannot be observed as these cells contain a so called nucleolus precursor body (NPB). The precise composition and function of this nuclear organelle is currently unknown. With respect to the low transcriptional activity of the fully-grown oocytes, it is clear that the NPB does not serve as the site of massive ribosome production. Thus, it is probable that the NPB serves as the site of sequestration and storage of original nucleolar proteins. At the same time, it is well established that the nucleolus in somatic cells is actually a multifunctional nuclear organelle that isfor example involved in cell cycle or epigenetic regulation. Our previous results indicate that the NPB might be in fact also a multifunctional organelle. In the proposed project, we would like to characterize in more detail the composition of the NPB and this in turn will help to establish more precisely its function.