Passage: 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.

Question: Based on this passage, the nucleoli found within the nucleus are the sites of ____ synthesis.​?
Steam of consciousness below:

To answer the above question, the relevant sentence is: It is best known as the site of ribosome biogenesis.
So, the answer is ribosome.