R-loops, three-stranded nucleic acid structures composed of an RNA:DNA hybrid and a displaced single-stranded DNA, are ubiquitously present from bacteria to human. The RNA sequence in R-loop is usually transcribed in cis, and R-loops generated with non-coding RNAs in trans have also been reported.

Initially considered as rare by-products of transcription, R-loops are now found widely distributed across the genome. Excessive R-loops are critical sources of genome instability, and underlie many human diseases, such as cancers, neurodegenerative disorders, and autoimmune diseases. Intriguingly, R-loops have been increasingly appreciated as key cellular regulators in many physiological processes, including DNA replication, homologous recombination, DNA damage repair, transcription and etc. To maintain R-loops at physiological level, cells have evolved to employ multiple proteins to regulate R-loops. Increasing number of proteins are now suggested to regulate R-loop dynamics, including RNA processing proteins and DNA damage-related factors to counteract R-loop formation, and nucleases and helicases to resolve existing R-loops. Collectively, the study of R-loop functions and its regulation will greatly advance both basic and translational research.