A one-year-old girl is completely cancer free, after doctors used "designer immune cells" for the first time in history in a desperate attempt to save her life.

Layla Richards was diagnosed with lymphoblastic leukemia when she was just 14 weeks old. After chemotherapy, bone marrow transplants, and various experimental treatments failed, doctors told the parents Layla would die.

That was five months ago, and it's when researchers at University College in London stepped in. The family having refused to give up, agreed to a treatment never before tried on humans.

A one-year-old girl is completely cancer free, after doctors used "designer immune cells" for the first time in history in a desperate attempt to save her life.

Layla Richards was diagnosed with lymphoblastic leukemia when she was just 14 weeks old. After chemotherapy, bone marrow transplants, and various experimental treatments failed, doctors told the parents Layla would die.

That was five months ago, and it's when researchers at University College in London stepped in. The family having refused to give up, agreed to a treatment never before tried on humans.

They received special permission from the British government and Great Ormond Street Hospital (GOSH) for Children to try a genetic treatment that had only been tested in a lab.

It's called "genome editing" — creating and transplanting special cells into the body to fight cancer.

The body normally relies on T-cells to fight infection, but those don't always recognize leukemia cells. So doctors took T-cells from a donor, altered them with splices of other genes so they would only see leukemia cells — and so that Layla's body wouldn't reject the transplanted cells.

Then they turned loose the edited cells in Layla's body.

Within months, the little girl's leukemia was completely gone.

"If replicated, it could represent a huge step forward in treating leukemia and other cancers," Professor Waseem Qasim, Professor of Cell and Gene Therapy at University College London, and an immunologist at GOSH said.

Doctors point out that Layla was a "very strong" little girl, and it's unknown if they can repeat her success with other patients.

The genome editing researchers published the results of Layla's case in the journal Nature.

(WATCH the video above from NBC News)