DNA undergoes simple chemical modifications – methylation. These modifications occur in one of its four bases: cytosine (C). Accordingly, cytosine naturally occurs in two, alternative forms: “methylated” or “unmethylated”. The consequences of differential methylation lie in the regulation of gene activity: methylated cytosines in the promotor region of a gene lead to inactivation, while unmethylated promotors allow gene activation and expession. Thus, methylation acts as an "on" and "off" switch for genes. Accordingly, methylation signals at selected positions are strongly correlated with cellular properties and the signals of a specific selection of methylation position (“the fingerprint”) is characteristic of particular cells and cell types. These fingerprints allow an accurate determination of the the identity of any cell. Consequently, DNA methylation can serve as a robust quality control tool for cellular products and should, eventually, become the gold standard technology for regulatory approval of cellular products.
