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Genes for ribosomal RNA are clustered on the short arms of chromosomes 13, 14, 15, 20, 21. The gene for RNR1 exists in multiple copies on chromosome 13. Each gene cluster contains 30-40 copies and encodes a 45S RNA product that is then cleaved to form 18S, 5.8S and 28S rRNA subunits. In general genes for RNA remain poorly annotated in most large public databases.