Do all the founders that inherited the transgene express the transgene?
No, the chromosomal environment at the transgene integration site can significantly influence transgene expression, a phenomenon called position effect variegation. For example, if the transgenic DNA integrates into heterochromatin, which is transcriptionally inactive, the transgene will not be expressed in the adult mouse. As integration is a random event, transgenic expression levels and patterns can vary substantially among founders. Sequences within the transgene can also effect expression. Vector sequences of 100bp or more may be recognized as foreign and target the locus for transcriptional inactivation, whereas the inclusion of heterologous introns or insulator sequences can enhance expression.
