The silencing of a gene is most often accomplished via methylation of the DNA. The methyl groups are added to the gene's promoter region and thus, the DNA is not read by transcriptional enzymes.

In order to silence a gene by methylation, methyl groups are added to the promoter region of DNA. This area is upstream of the coding sequence and is responsible for initiation of transcription. Thus, methylating the promoter region inhibits further transcription of the gene.

The only two bases that can be methylated are cytosine and adenine.

Alkylating agents and can also cause cancer, but they lead to methylation and mismatch mutations rather than the formation of pyrimidine dimers.

A null mutation is one that deactivates a gene entirely. Point mutations are those that occur within a single, small site in a gene. Inversion involves the reversal of orientation of a DNA segment. Deletion occurs when a whole part of a chromosome is removed, joining two ends that were far apart. Translocation involves the exchange of genetic material from two chromosomes that are not homologous.

Unmethylated cytosine spontaneously deaminates to uracil. Over time, methylated cytosine is spontaneously deaminated to thymine. Random deamination of methylcytosines causes mutation, creating hot spots. The vast majority of DNA methylations in mammals occurs at CpG (cytosine-phospate-guanine) sites.