<Table> <Tr> <Td> </Td> <Td> This article may be too technical for most readers to understand . Please help improve it to make it understandable to non-experts, without removing the technical details . (October 2012) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article may be too technical for most readers to understand . Please help improve it to make it understandable to non-experts, without removing the technical details . (October 2012) (Learn how and when to remove this template message) </Td> </Tr> <P> The nucleolus (/ nj uː ˈkliːələs, - kliˈoʊləs /, plural nucleoli / - laɪ /) is the largest structure in the nucleus of eukaryotic cells . It is best known as the site of ribosome biogenesis . Nucleoli also participate in the formation of signal recognition particles and play a role in the cell's response to stress . Nucleoli are made of proteins, DNA and RNA and form around specific chromosomal regions called nucleolar organizing regions . Malfunction of nucleoli can be the cause of several human conditions called "nucleolopathies" and the nucleolus is being investigated as a target for cancer chemotherapy . </P> <P> The nucleolus was identified by bright - field microscopy during the 1830s. Little was known about the function of the nucleolus until 1964, when a study of nucleoli by John Gurdon and Donald Brown in the African clawed frog Xenopus laevis generated increasing interest in the function and detailed structure of the nucleolus . They found that 25% of the frog eggs had no nucleolus and that such eggs were not capable of life . Half of the eggs had one nucleolus and 25% had two . They concluded that the nucleolus had a function necessary for life . In 1966 Max L. Birnstiel and collaborators showed via nucleic acid hybridization experiments that DNA within nucleoli code for ribosomal RNA . </P>

The nucleoli found within the nucleus are the sites of ____ synthesis.​