When foreign genes are inserted into plants, they try to silence them, a phenomenon known as transgene silencing. Scientists have found that plants use the same mechanisms to silence foreign genes as to regulate their own genes both during development and in response to environmental stresses. These silencing mechanisms are epigenetic, that is they do not alter the DNA sequence of genes, and have the advantage of being reversible. Peter is currently testing the hypothesis that an epigenetic mechanism called antisense transcription controls the levels of petunia cytokinin-producing enzymes, proteins whose concentrations vary between tissues and at different developmental stages.