Episome

As of 1999, there were many known sequences of DNA (deoxyribonucleic acid) that allow a standard plasmid to become episomally retained. One example is the S/MAR sequence.

The length of episomal retention is fairly variable between different genetic constructs and there are many known features in the sequence of an episome which will affect the length and stability of genetic expression of the carried transgene. Among these features is the number of CpG sites which contribute to epigenetic silencing of the transgene carried by the episome.

Mechanism of episomal retention

The mechanism behind episomal retention in the case of S/MAR episomes is still uncertain. As of 1985, in the case of latent Epstein-Barr virus infection, episomes seemed to be associated with nuclear proteins of the host cell through a set of viral proteins.