The nucleolus organizer regions (NORs) comprise tandem arrays of several hundred ribosomal DNA (rDNA) genes that code for the major rRNA elements. These elements are transcribed but not translated, and serve as structural elements of the ribosomes. The rDNA clusters are separated by non-transcribed spacers (NTSs): post-transcriptional processing removes the NTS elements.   NORs are distinctive cytological features of the nucleus, and appear as darkly staining heterochromatic regions. As shown by the comparison of frog and fly NOR regions above, the basic arrangement is highly conserved between vertebrate and invertebrate genomes.