Nucleolus organizer region

Nucleolus organizer region (NOR) is a chromosomal region around which the nucleolus forms. It is also called the nucleolar organizer.

The region contains several tandem copies of ribosomal RNA genes. In humans the NOR contains genes for 5.8S, 18S, and 28S rRNA clustered on the short arms of chromosomes 13, 14, 15, 21 and 22 (the acrocentric chromosomes).