Natural antisense transcripts.

The sequencing of whole genomes and the subsequent annotation of cDNAs revealed that about 20% of human and mouse genes overlap resulting in potential pairs of sense and antisense transcripts. An increasing number of experimentally identified antisense transcripts concur with this predication. Characterization of overlapping transcripts in various species indicates that this form of RNA-mediated gene regulation represents a widespread phenomenon. However, the physiological relevance of natural antisense transcripts remains obscure. Genomic studies suggest that duplex formation between sense and antisense is required for biological function. Antisense transcripts play an established role in imprinting and X-chromosome inactivation and genomic rearrangements as observed in B and T leukocytes. Only a relatively small percentage of the predicted antisense transcripts are related to these biological phenomena that are also related to mono-allelic expression. Consequently, there are at least two categories of natural antisense transcripts that show significant differences with regard to their biological function as well as the potential mechanisms involved.