RNA, ribosomal 4 encoding the rRNA components of the ribosome are organized as tandem rDNA repeating units, designated RNR1 through RNR5, in the p12 region of chromosomes 13, 14, 15, 21 and 22. Each rDNA unit encodes a 45S rRNA which serves as the precursor for an 18S, 5.8S and 28S rRNA. The number of rDNA repeating units varies between individuals and from chromosome to chromosome, although usually 30 to 40 repeats are found on each chromosome. This ribosomal repeating unit located on chromosome 21 is not currently annotated on the reference genome. (provided by RefSeq)