Localization of a gene: the nucleolar organizer.

Nucleolus differentiates around the nucleolus organizer regions of the chromosomes or NORs. In the interphasic nucleoli the fibrillar centers are now considered as the NORs. The purpose of this editorial is to review the experimental data which allowed such identification. Our current concepts regarding this point result from three lines of evidence: 1) specific localization during nucleologenesis, 2) in situ hybridization with labeled ribosomal RNA or DNA, and 3) specific staining with silver.