The nucleolus ( or , plural nucleoli or ) is the largest structure in the nucleus of eukaryotic cells, where it primarily serves as the site of ribosome synthesis and assembly. Nucleoli also have other important functions like assembly of signal recognition particles and playing a role in the cell's response to stress. Nucleoli are made of proteins and RNA and form around specific chromosomal regions. Malfunction of nucleoli can be the cause of several human diseases.