Repetitive sequences occupy a huge fraction of essentially every eukaryotic genome Repetitive sequences cover more than 50% of mammalian genomic DNAs, whereas gene exons and protein coding sequences occupy only similar to 3% and 1%, respectively Numerous genomic repeats include genes themselves They generally encode "selfish" proteins necessary for the proliferation of transposable elements (TEs) in the host genome The major part of evolutionary "older" TEs accumulated mutations over time and fails to encode functional proteins However, repeats have important functions also on the RNA level Repetitive transcripts may serve as multifunctional RNAs by participating in the antisense regulation of gene activity and by competing with the host encoded transcripts for cellular factors In addition, genomic repeats include regulatory sequences like promoters, enhancers, splice sites, polyadenylation signals, and insulators, which actively reshape cellular transcriptomes TE expression is tightly controlled by the host cells, and some mechanisms of this regulation were recently decoded Finally, capacity of TEs to proliferate in the host genome led to the development of multiple biotechnological applications