DDX21 and breast cancer: Notably, recent research confirmed that DDX21 specifically bound to nucleolar small RNA (snoRNA) to localize on nucleoli and then affected the RNA polymerase I (Pol I) complex including POLR1A, POLR1B to regulate rDNA transcription and promoted ribosomal organisms, ribosome biogenesis, and protein translation, thereby promoting cell proliferation in breast cancer [11, 12].