Gene therapy vs. Dominant dissease.?

Hi.

I am writing a term-paper about cancer treatment by gene therapy and I'd like to know a few things.

While writing my paper I thought that although gene therapy could introduce a working wild-type gene into a cell with a defective one, this would only help if the defective one did not have an adverse effect (for instance in case of non-functioning p53) however I saw no "easy" way how to avoid those genes that code for defective proteins that are always on and thus inducing the cancerous phenotype (such as Ras.)

Is there a walk around on this (I thought about adding a second gene that codes for a inhibitor protein though that would be extremely difficult to do, or just using gene therapy in conjunction with siRNA or perhaps any other drug.)

Please any insight would be appreciated. If plausible link some source so I can use it in my paper as reference.

Thank you.