Chromosomal sequences flanking an efficiently expressed transgene dramatically enhance its expression.

The expression of transgenes in mice is influenced strongly by their site of integration in the genome. To test whether the chromosomal sequences immediately flanking a site of integration could positively influence expression we isolated the 5' and 3' chromosomal sequences from an efficiently expressed transgenic locus. These chromosomal sequences were incorporated into transgene constructs and these were then introduced into mice. Linking them to the original transgene dramatically enhanced its expression and conferred a degree of position independent expression upon it. However, the results were not as marked when these sequences were linked to other constructs, showing that the effectiveness of such flanking chromosomal sequences is highly dependent on the nature of the transgene used.