Figure 1.

Variation in the level of proviral expression within cell clones might be accounted for by the degree of methylation. In this model the degree of repression, set up at the time of integration, is critical to the degree of gene expression. Studies supporting this model include [36,47,49,50]. DNA methylation is an attractive candidate as a molecular correlate of repression and is depicted as such here and in Figure 2. There is, however, evidence suggesting that other molecular mechanism may be involved (see text). (a) Provirus integrated into repressive chromatin is stably repressed, probably correlating with a high degree of proviral DNA methylation. (b) Provirus in partially repressed chromatin is unstable and may proceed to become tightly repressed, or continue to be expressed but could be induced to a higher degree of expression. The change in epigenetic mark could arise from cell division. (c) Integration into permissive chromatin leads to high-level gene expression.