DNA methylation has been implicated in mammalian development. Transcription units contain CpG islands, but expression of CpG island associated genes in normal tissues was not believed to be controlled by DNA methylation. There are, however, numerous CpG islands containing tissue-dependent and differentially methylated regions (T-DMR), which are potential methylation sites in normal cells and tissues. Genomic scanning which focused on T-DMRs in CpG islands revealed that the DNA methylation profile of each cell/tissue is more complicated than previously considered. Differentiation of cells is associated with both methylation and demethylation, which occur at multiple loci. The epigenetic system characterized by DNA methylation requires cells to memorize gene expression patterns, thus, standardizing cellular phenotypes.