Figure 1.

Classes of epialleles. (a) Epigenetic modifications and associated silencing of the adjacent gene is dependent on a specific cis-element, often a repetitive element. (b) Epigenetic modification is triggered by another locus or allele. Once the modification has been established, the trigger is no longer required for its maintenance. (c) Epigenetic modification is triggered by another locus or allele, but the trigger is permanently required.