The American and Canadian biologists have tried to find out what affects the size of the nucleolus. As a result of the experiments carried out on yeasts, there have been discovered 113 genes, whose mutation caused phenotypic changes of the nucleolus. With regards to the case with drosophila, 757 genes were responsible for resizing of the nucleolus. The increase in the size of the nucleolus was caused by a mutation in the genes, which was responsible for the regulation of the cell cycle, processing of the ribosome and matrix RNA. As to loss of function of proteins, which participate in the fundamental processes such as vesicular transport in a Golgi complex from the endoplasmic reticulum, R-RNA synthesis, building of nucleosomes, histone transcription and acetylation regulation, it led to the phenotypically decreased nucleolus.