This year, 23,000 people will be told they have brain cancer and 13,000 of those people will die. But now, a gene that kills itself is helping destroy life-threatening tumors and save patients' lives.

Right after her daughter, Sloan, was born, Marni Kass started feeling strange. She thought she was suffering from postpartum depression, but what she had was a brain tumor. She had surgery to remove it, and chemotherapy and radiation to kill what was left. But it wasn't enough. She was given four months to live and now, three years later, she's still fighting—thanks to a virus injected into her brain.

"When we inject, the virus infects the cells around it, then makes more of itself and spreads to the next cells," Dr. Santosh Kesari, a neuro-oncologist at UC San Diego's Moores Cancer Center.

The virus, called Toca 511, carried a suicide gene to cancer cells. It keeps replicating as it finds more cancer cells. After four weeks, patients are given a drug that's activated by the suicide gene.

"That gene converts it to a chemotherapy drug," Kesari said.

The drug delivers a toxic dose to the cancer cells without harming healthy cells in the brain.

Dr. Kesari thinks using suicide genes could someday be the norm for treating all different types of cancer—which could mean there wouldn't be the need for open surgeries, and instead, just a biopsy to deliver cancer-fighting drugs.