Aberrant DNA methylation at the 5-position of cytosine, catalyzed by DNA methyltransferases
(DNMTs), is associated with not only various cancers by silencing of tumor suppressor genes but
also other diseases. The DNMTs, especially the DNMT1, DNMT3A and DNMT3B, are often overexpressed
in various cancer tissues and cell lines. DNMTs are important epigenetic targets for drug development
since the DNA methylation is reversible. This review summarizes an array of nucleoside and
non-nucleoside inhibitors of DNMTs, as well as their biological activities. Among these inhibitors, the
nucleoside analogue azacytidine and its deoxy derivative decitabine are both irreversible DNMT inhibitors
and approved for the treatment of myelodysplastic syndrome.