Huang, X. et al. DNA Cell Biol. advance online publication (7 February 2012).

Epigenetic changes can affect gene regulation at mammalian genomic loci. For instance, inappropriate demethylation of the widespread Line-1 retroelement has been associated with cancer gene expression. Huang et al. developed a non-integrating reporter to assay DNA accessibility at targeted loci in living cells. The biosensor consists of two independent zinc-finger proteins that recognize distinct 15-base-pair sequences in Line-1, bringing two halves of a luciferase reporter together. To help reduce background, each luciferase domain is joined to half of an intein, which excises when the parts are brought together. When both constructs are expressed in HeLa cells, the sensor shows brighter luminescence in the presence of DNA-demethylating drugs, suggesting increased access of the probe for binding in the absence of demethylation.