Human DHX9 helicase preferentially unwinds RNA-containing displacement loops (R-loops) and G-quadruplexes

► In this study we show that the essential helicase DHX9, also known as RNA helicase A and nuclear DNA helicase II. ► Efficiently dissolves DNA and RNA containing displacement loops (D-loops and R-loops). ► Efficiently removes guanine containing tetraplexes (G4 structures) from both DNA and RNA strands. ► Therefore, DHX9 may be particularly suited to cotranscriptionally remove RNA containing secondary structures and, in consequence, speed-up transcription and increase genome stability.