Abbreviated DCR, these DNA sequences are unusual in that they allow gene expression even in tissues that do not normally express the chromosome they are located in. Thus, they are very similar to tissue-specific enhancers, but they differ in that DCRs isolate adjacent chromatin from the effect of the host integration site. So, they can manifest a degree of exogenous gene expression which is dependent upon the exogenous gene copy number. Since DCR has been identified to within a few kilobases, it is possible to construct retroviral vectors with DCR. This could potentially be used to correct genetic defects by replacing defective genes. They're kind of like the override switch, they let you do things that wouldn't ordinarily occur in that cell. Like making an erythroid cell produce a protein normally only found somewhere else in the body.