Anti-DExD-box helicase 21 antibodies are used in the immunodetection of the protein encoded by the DDX21 gene. In humans, the canonical protein has a reported length of 783 amino acid residues and a mass of 87.3 kDa. Its subcellular localization is in the nucleus, mitochondria, and cytoplasm. Alternative splicing is reported to yield 2 different isoforms for this protein. A member of the DEAD box helicase protein family, it is reported to be a RNA helicase that acts as a sensor of the transcriptional status of both RNA polymerase (Pol) I and II: promotes ribosomal RNA (rRNA) processing and transcription from polymerase II (Pol II). Post-translational modifications have been described, including acetylation. Other names for this target antigen include GURDB, II/Gu, RH, RH II/Gu, RH-II/GU, RH-II/GuA, gu-alpha, and GUA.