In molecular biology, U21 is a member of the C/D class of snoRNA which contain the C (UGAUGA) and D (CUGA) box motifs.

U21 is encoded within an intron of the gene for ribosomal protein L5 in mammals,[1] but within introns of the ADP ribosylation factor gene in Drosophila.

[2] U21 snoRNA has a 13 nucleotide region of complementarity with an invariant region of eukaryotic 28S ribosomal RNA.

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