DNA methylation involves the addition of a methyl group to the 5 position of cytosine (one of the four bases of DNA), which occurs in the context of CpG (cytosine followed by guanine) dinucleotides. This modification can be inherited through cell division. DNA methylation is typically removed during zygote formation and reestablished through successive cell divisions during development. DNA methylation is a crucial part of normal organismal development and cellular differentiation in higher organisms. DNA methylation stably alters the gene expression pattern in cells such that cells can "remember where they have been"; in other words, cells programmed to be pancreatic islets during embryonic development remain pancreatic islets through out the life of the organism without continuing signals telling them that they need to remain islets. In addition, DNA methylation suppresses the expression of viral genes and other deleterious elements which have been incorporated into the genome of the host over time. DNA methylation also forms the basis of chromatin structure, which enables cells to form the myriad characteristics necessary for multicellular life from a single immutable sequence of DNA. DNA methylation also plays a crucial role in the development of nearly all types of caner.