Most of this review has focused on changes to the DNA sequence, such as SNPs and copy number variants. The emerging field of epigenetics assesses the impact of chemical modification of the DNA bases rather than changes to the sequence itself. To better understand this concept, one needs to understand that the expression of a gene is influenced by transcription factors, which bind to specific sequences of DNA. It is through the binding of transcription factors that genes can be turned on or off. Epigenetic mechanisms involve changes to how readily transcription factors can access the DNA. Several different types of epigenetic changes are known to exist that involve different types of chemical changes that can regulate DNA transcription. One epigenetic process that affects transcription binding is DNA methylation. DNA methylation involves the addition of a methyl group (CH3) onto a cytosine (one of the four base pairs that make up DNA). This leads to gene silencing because methylated DNA hinders the binding of transcription factors. A second major regulatory mechanism is related to the configuration of DNA. DNA is
