RNA:DNA hybrids serve as primers for DNA replication, regulate gene expression and are structural components of specific eukaryotic genomic elements, such as telomeres, centromeres and ribosomal DNA. RNA:DNA hybridization is also a crucial step in cell-specific processes, such as immunoglobulin class-switch recombination (CSR) in activated B cells and CRISPR/Cas9-mediated adaptive immunization in prokaryotic cells.