X inactivation, differentiation, and DNA methylation

Cytogenet Cell Genet. 1975;14(1):9-25. doi: 10.1159/000130315.


A model based on DNA methylation is proposed to explain the initiation and maintenance of mammalian X inactivation and certain aspects of other permanent events in eukaryotic cell differentiation. A key feature of the model is the proposal of sequence-specific DNA methylases that methylate unmethylated sites with great difficulty but easily methylate half-methylated sites. Although such enzymes have not yet been detected in eukaryotes, they are known in bacteria. An argument is presented, based on recent data on DNA-binding proteins, that DNA methylation should affect the binding of regulatory proteins. In support of the model, short reviews are included covering both mammalian X inactivation and bacterial restriction and modification enzymes.

Publication types

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

MeSH terms

  • Animals
  • DNA (Cytosine-5-)-Methyltransferases / metabolism
  • DNA / metabolism*
  • DNA Replication
  • Methylation
  • Models, Molecular
  • Sex Chromatin
  • Sex Chromosomes / metabolism*


  • DNA
  • DNA (Cytosine-5-)-Methyltransferases