DNA methylation is a postreplicative modification occurred in most prokaryotic 
and eukaryotic genomes, which has a variety of important biological functions 
including regulation of gene expression, gene imprinting, preservation of 
chromosomal integrity, and X-chromosome inactivation. According to their 
structure and functions, DNA methyltransferases (Dnmts) are divided into two 
major families in mammalian cells: maintenance methyltransferase (Dnmt1) and de 
novo methyltransferases (Dnmt3a, Dnmt3b, and Dnmt3L). In addition, Dnmt2 also 
displays weak DNA methyltransferase catalytic activity, but newly founded 
function is to methylate cytosine 38 in the anti-codon loop of tRNAAsp. These 
Dnmts are crucial for mammalian growth and development. Dnmts deficiency will 
lead to embryonic development defects, cancer, and other diseases. Therefore, 
Dnmts could be important therapeutical targets. This article summarizes the 
classification, function, and recent research progress in DNA 
methyltransferases.