Transcriptional repression mechanisms of nucleolus organizer regions (NORs) in humans and chimpanzees.

Polymorphisms related to transcriptional inactivation of nucleolus organizer regions (NORs) have long been described in many animals, particularly humans. However, the precise aetiology of such variations is not always clear. We conducted analyses to investigate the repression mechanisms in humans and chimpanzees using FISH (fluorescence in situ hybridisation) with 18S rDNA, Ag-NOR (silver nitrate) staining, C-banding, and the in situ nick translation technique with the HpaII restriction enzyme. Examination of 48 humans and 46 chimpanzees suggested that there are at least three different mechanisms that produce inactivation of NORs. These include: (1) elimination of rDNA; (2) DNA methylation: (3) gene silencing due to position effects induced by heterochromatin (C-bands) and/or telomeres.