Methylation is important for turning on and off mammalian DNA. This is true for silencing viral DNA in the body as well as cellular DNA. There are sections of the DNA prior to the information or coding portion that contain regulatory sequences or sections. When these regulatory regions are methylated properly they turn on and off the information portions, as they should. During development, DNA methylation patterns are established and are essential for normal development. During new cell synthesis these patterns are then replicated. When these regions do not have the correct amount of methyl groups bound to them it can prevent the information from being turned off, resulting in autoimmunity, aging and cancer.