Ribosomal RNA (rRNA) is transcribed as precursor molecules that are processed into the mature rRNAs that make up parts of ribosomal subunits. In eukaryotes, two long precursor rRNA molecules are transcribed, one producing 5S rRNA and the other producing 28S, 5.8S and 18S rRNAs. rRNA processing involves cleavage of the precursor molecules by ribonucleases to remove spacer sequences and produce the functional rRNAs, which occurs through a series of cleavage steps in the nucleolus that generate intermediates before the final mature rRNAs are released.