CpG islands are areas of CpG sites in close proximity. Due to the ability to methylate cytosines, CpG sites can be used to control gene expression by methylation and demethylation.
Methylation of CpG sites reduces gene expression, due to the DNA becoming less transcriptionally active and more tightly associated with histone proteins. By controlling the methylation state of CpG sites, the cell is able to modulate gene expression.
This is significant, because it allows the cell to control gene expression, allowing the subsequent control of how the cell directs its energy and metabolic flux. Histone deacetylases (HDACs) and histone acetylases (HATs) are enzymes able to remove and add acetyl groups, respectively.