Problems Due to Gene Silencing - Transgene silencing results in variable and, usually, unacceptable levels of transgene expression. This necessitates an extensive and often prolonged evaluation of transgenic lines. It may often become necessary to order unstable transgenic lines in hybridization and selection programmes in order to develop transgenic lines with stable and the desired level of transgene expression.

Both these steps require additional resources and, above all, time, and delay the release of transgenic lines as varieties. Finally, instability would cause problems in the registration of the varieties as they are required to be distinct, uniform and stable (DDS).