ABSTRACT

Particular mouse strains are associated with different forms of transgene behavior. Among the epigenetic phenomena, position effects are the simplest to overcome because the genome has its own mechanisms to avoid clashes between genes in different chromatin environments, and these mechanisms can be harnessed to protect integrated transgenes. In addition, transgene integration may occasionally cause unexpected changes in phenotype resulting from serendipitous rearrangements of the host genome. Arrays are still generated but they are much smaller than the transgenome, and are often organized as simple direct repeats. In the context of gene transfer experiments, position effects are defined as influences on transgene activity brought about by environmental differences between different integration sites. The loss of transgene expression in transformed cells and transgenic animals is generally termed transgene silencing. Vector DNA can pose its own problems because certain bacterial sequences in transgenic animals have been shown to exert specific negative transcriptional effects on adjacent genes.