RNA in ribosomes accounts for most of the RNA in human cells, and most of the transcription activity of the nucleus. Ribosomal RNA gene transcription is tightly regulated in order to provide the proper amount of rRNA for ribosome assembly, according to the cellular need for protein synthesis. It is related to the function of the nucleolus, whose position in the nucleus is determined by the position of a discrete chromosomal locus known as a nucleolus organizer region (NOR) . The NOR is a loci at which repetitive, nearly identical, rRNA genes, several hundred in number, are clustered in long tandem arrays, typically spanning millions of basepairs. Each rDNA sequence encodes a precursor transcript that is processed to generate one molecule each of 18S, 5.8S and 28S rRNA. During the transcription of rDNA, the elongating nascent rRNA transcripts create a structure called Christmas tree. Some rRNA genes (rDNA) are silenced by epigenetic mechanisms. Some of this silencing is inherited on cell division and this has allowed understanding of euchromatic, i.e. transcriptionally active, and the heterochromatic, i.e. transcriptionally silent DNA. DNA methyltransferases and histone-modifying enzymes work in concert with chromatin-remodeling complexes and RNA-guided mechanisms to establish a specific chromatin structure that defines the transcriptional state of rDNA genes. In man, like all mammals there is a nucleolar remodelling complex (NoRC, involves TIP5 (TTF-1-interacting protein 5) and the ATPase SNF2 h). NoRC binds to the rDNA promoter of silent genes and represses rDNA transcription through histone-modifying and DNA-methylating activities. The energy-dependent nucleolar silencing complex (eNoSC, involves NAD(+)-dependent histone deacetylase SIRT1, a histone methyltransferase SUV39H1, and nucleomethylin) provides more immediate flexiability to the rDNA on/off switch. . Altered NAD(+)/NADH ratio in response to glucose starvation regulates the silencing activity of eNoSC. Another key mechanism involving rRNA is nucleolar dominance in which one parents NOR is suppressed.