Insulators protect transgenes from chromosomal position effects. Production of transgenic animals can depend upon the ectopic integration of transgenes randomly into a eukaryotic genome. In this example, a transgene carrying the Drosophila white gene (oval and rectangle) flanked by gypsy insulators (triangles) was integrated at three distinct genomic locations (shown by arrows pointing to the Drosophila polytene chromosomes). The white transgene encodes a protein required for eye pigmentation, with high levels of white expression leading to the production of red eyes. Transgenic flies with insulator function display a uniform pigmentation at all three locations (with insulator). A loss of insulator function (no insulator) caused by crossing the transgenes into genetic backgrounds mutant for the gypsy insulator proteins revealed that in two genomic locations the transgenes were integrated into repressive chromatin. Without the insulator, a spread of silent chromatin occurs into the unprotected white transgene, causing decreased white transcription and a loss of eye pigmentation.
