DDX21 and cancer: DExD/H box nucleic acid helicases including DEAD-box helicase 60 (DDX60), DEAH-box helicase 36 (DHX36), DEAD-box helicase 21 (DDX21), and DEAH-box helicase 9 (DHX9) are one group of the nucleic acid-sensing PRRs, as well, which are emerging as important regulators of many cellular processes in cancers (6).