Position Effect Variegation
When a euchromatic gene is placed close to heterochromatin, heterochromatin stochastically spreads onto the gene and represses the expression. Once heterochromatin is established on the gene, it is stably maintained in many cell cycles but at a certain frequency heterochromatin shrink back and the gene expression starts again. Consequently, the expression pattern of the gene expression forms a kind of mosaic pattern. This stochastic variegation of gene expression from genes place close to heterochromatin is called Position Effect Variegation (PEV).