Cytosine methylation and the ecology of intragenomic parasites

Trends Genet. 1997 Aug;13(8):335-40. doi: 10.1016/s0168-9525(97)01181-5.


Most of the 5-methylcytosine in mammalian DNA resides in transposons, which are specialized intragenomic parasites that represent at least 35% of the genome. Transposon promoters are inactive when methylated and, over time, C-->T transition mutations at methylated sites destroy many transposons. Apart from that subset of genes subject to X inactivation and genomic imprinting, no cellular gene in a non-expressing tissue has been proven to be methylated in a pattern that prevents transcription. It has become increasingly difficult to hold that reversible promoter methylation is commonly involved in developmental gene control; instead, suppression of parasitic sequence elements appears to be the primary function of cytosine methylation, with crucial secondary roles in allele-specific gene expression as seen in X inactivation and genomic imprinting.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.
  • Review

MeSH terms

  • Animals
  • Cytosine / metabolism*
  • DNA Methylation*
  • DNA Transposable Elements*
  • Gametogenesis
  • Gene Expression Regulation, Developmental*
  • Mammals / genetics
  • Neoplasms / genetics
  • Retroviridae / genetics


  • DNA Transposable Elements
  • Cytosine