The maze of paramutation: a rough guide to the puzzling epigenetics of paramutation.

Epigenetic mechanisms maintain gene expression states through mitotic and sometimes meiotic cell divisions. Paramutation is an extreme example of epigenetic processes. Not only an established expression state is transmitted through meiosis to the following generations but also an information transfer occurs between alleles and leads to heritable changes in expression state. As a consequence the expression states can rapidly propagate in population, violating Mendelian genetics. Recent findings unraveled an essential role for siRNA-dependent processes in paramutation. Despite significant progress, the overall picture is still puzzling and many important questions remain to be answered.