ABSTRACT

One of the more interesting phenomena regarding transgene methylation is that the extent to which the transgene is methylated is sometimes found dependent on the sex and/or strain of the parent which donated the transgene. Most transgenes can be designed to include sufficient regulatory sequence information to confer, in the transgenic animal, appropriate developmental and tissue-specific expression. While some of these variable effects on expression can be associated with differential methylation of the transgene in single lines of transgenic animals, many of the effects are known to be due to the specific chromosomal environment at the site of transgene integration and are known as position effects. Position effects are now recognized to be a universal phenomenon amongst eukaryotes. Evidence derived from many experimental approaches over the past ten to fifteen years suggests that there are a variety of mechanisms by which DNA methylation represses transcriptional activity.