Stochastic epigenetic silencing of retrotransposons: does stability come with age?

It has been hypothesized that phenotypic variation in mammals could in part be due to incomplete and variable silencing of retrotransposons in somatic cells. This theory is based on the fact that some recent endogenous retroviral (ERV) insertions in the mouse exert variable effects on genes in isogenic animals, depending on the variable state of ERV methylation. In this article, we review the evidence for this and related phenomena and suggest that such stochastic epigenetic silencing is restricted to very recent insertions. We also present a model to explain the acquisition of a more stable epigenetic state for transposable element insertions through time.