Dictionary.com
definitions

DNA methylation

[mĕth′ə-lā′shən]

The modification of a strand of DNA after it is replicated, in which a methyl (CH3) group is added to any cytosine molecule that stands directly before a guanine molecule in the same chain. Since methylation of cytosines in particular regions of a gene can cause that gene's suppression, DNA methylation is one of the methods used to regulate the expression of genes.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
dna methylation in Culture

DNA methylation

[(meth-uh-lay-shuhn)]

The chemical reactions that place a methyl group (a combination of one carbon atom and three hydrogen atoms) at a particular spot on DNA during organismal development. The effect of this process is probably to “turn off” various genes during the process of cellular differentiation, causing the cell to develop into a specific type.

Note

It is thought that during cloning, the methyl groups are removed from the DNA, turning the genes back “on” again.

Note

There is evidence that embryonic stem cells are cells in which this process has not yet occurred.
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.