Intrexon, a Blackburg, Va., developer of engineered gene therapies it calls “transcriptional therapeutics,” raised $25 million in a third round of financing provided by Third Security. Earlier funding rounds were provided by Third Security and NewVa Capital Partners.

Intrexon is at work on drugs based on stretches of DNA that can integrate themselves into the genomes of individual cells and begin to function like native genes. These newly installed genes could be turned on or off by “activator” molecules such as another drug, potentially allowing the controlled production of new proteins that affect other cellular processes. Intrexon intends to direct the technology against cancer.

Such gene therapy has long been tantalizing, although it’s never worked particularly well in practice. The company’s release is here.