Abstract: The genome is composed not only of genes but also of several noncoding functional elements (NOCs/ncFE, here I use NOCs), such as transcriptional promoters, enhancers, replication origins, centromeres and telomeres. rDNA has both gene and NOC characteristics. Thus, the rDNA encodes ribosomal RNAs, components of the ribosomes, that account for approximately 80% of the total RNA in a cell. However, rDNA may also act as a NOC with respect to cellular senescence by limiting the number of times a cell can divide. Here, I discuss how rDNA might function as a NOC to influence life span in a manner analogous to telomeres.CI - (c) 2011 The Author. Journal compilation (c) 2011 by the Molecular Biology Society of Japan/Blackwell Publishing Ltd.