A British teenage girl with an aggressive type of leukemia that experts claimed was “incurable” is now cancer free after receiving a first-of-its-kind treatment in which her DNA was modified to have her body create its own therapy, the BBC reports.

Doctors at National Health Service’s Great Ormond Street Hospital treated the 13-year-old girl with a new technology called “base editing” that allows them to literally alter specific sections of DNA to change the function of cells.  The patient suffered from T-cell acute lymphoblastic leukemia, a form of cancer that turns the body’s normally beneficial T-cells against the body.

Using donated T-cells, doctors used base editing, a technology developed just six years ago, to produce T-cells that would attack the patient’s cancerous T-cells.  Bases are what comprise genetic code.  There are four bases–adenine (A), cytosine (C), guanine (G) and thymine (T)–that are infinitely combined to produce our individual genetic codes. 

Doctors were able to go into a specific sector of the donor T-cell, change the base from one to another, and create a cell that attacked the cancerous cells, not replicate them.  While doctors treated the patient with the new T-cells, the patient also underwent chemotherapy to try to hold the cancer at bay, therefore the T-cell had to be modified further to make it immune to chemo.

Because the designer T-cells essentially destroyed her immune system, the patient underwent a bone-marrow transplant to jumpstart her immune system.  The transplant was successful, and she left isolation after a 16-week hospital stay.  Three months later, her scans show she’s cancer free.