RNR4 - RNA, ribosomal 45S cluster 4 Gene

45S ribosomal DNA (rDNA) arrays, or clusters, are present on human chromosomes 13, 14, 15, 21 and 22, designated RNR1 through RNR5, respectively. Each cluster consists of multiple 45S rDNA repeat units that vary in number among individuals and chromosomes, with total diploid copy number estimates ranging from 60 to >800 repeat units in a human genome. The 45S rDNA repeat unit encodes a 45S rRNA precursor, transcribed by RNA polymerase I, which is processed to form the 18S, 5.8S and 28S rRNAs. Gene and RefSeq, in collaboration with HGNC, currently describe one 45S rDNA cluster, and one set of 45S precursor and product rRNAs, for each of the five human chromosomes to which these loci are localized. This gene represents the 45S ribosomal DNA cluster on chromosome 21. [provided by RefSeq, Feb 2017]