An episome is a special type of plasmid, which remains as a part of the eukaryotic genome without integration.
Episomes manage this by replicating together with the rest of the genome and subsequently associating with metaphase chromosomes during mitosis.
Episomes do not degrade, unlike standard plasmids, and can be designed so that they are not epigenetically silenced inside the eukaryotic cell nucleus.
In 2004, it was proposed that non-viral episomes might be used in genetic therapy for long-term change in gene expression.
Among these features is the number of CpG sites which contribute to epigenetic silencing of the transgene carried by the episome.