Small nucleolar RNA SNORD21

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.