Cytosine DNA methylation (5-methylcytsone, 5mC) is the major DNA modification found in the genomes of animals and plants. Although the roles of 5mC and its oxidized derivatives in the regulation of gene expression are relatively well attested and extensively explored, a number of recent studies imply that noncytosine DNA modifications may also convey specific biological functions and act as "epigenetic" marks in multicellular organisms. Here we review experimental evidence for the presence of noncytosine epigenetic modifications in metazoans and plants focusing on two "unusual" DNA bases, 5-hydroxymethyluracil (5hmU) and N6-methyladenine (6mA), and suggest potential explanations for inconsistencies in the currently available data on abundance and potential biological roles of these DNA modifications in mammals.
Keywords: 5-hydroxymethyluracil; DNA modifications; Epigenetics; N6-methyladenine.