DNA methylation represents a form of genome annotation that mediates
gene repression by serving as a maintainable mark that can be
used to reconstruct silent chromatin following each round of replication.
During development, germline DNA methylation is erased in the
blastocyst, and a bimodal pattern is established anew at the time of implantation
when the entire genome gets methylated while CpG islands
are protected. This brings about global repression and allows housekeeping
genes to be expressed in all cells of the body. Postimplantation
development is characterized by stage- and tissue-specific changes in
methylation that ultimately mold the epigenetic patterns that define
each individual cell type. This is directed by sequence information in
DNA and represents a secondary event that provides long-term expression
stability. Abnormal methylation changes play a role in diseases,
such as cancer or fragile X syndrome, and may also occur as a function
of aging or as a result of environmental influences.