Two infants diagnosed with an aggressive and previously incurable form of leukemia are now in remission, after British doctors say they cured the babies using so-called “designer cells.”

In each case, scientists engineered a type of immune cell called a T cell to attack cancer cells. Each of the two infants, aged 11 and 16 months, had undergone many previous treatments, and all of them had failed. One of the infants, Layla Richards, attracted press attention in the fall when scientists at Great Ormond Street Hospital announced that Richards, who was diagnosed with cancer at just three months old, was vastly improved just a few months after beginning treatment. But at the time, scientists were hesitant to say that she had been cured. Now, the scientists say that Richards is in remission—and that they have cured a second baby as well, lending more evidence to the therapy’s effectiveness.

Similar treatments using engineered T-cells have been highly successful against blood cancers in other studies. But those studies rely on a patients own blood cells being altered. Using donated cells offers the potential for a cheaper, faster cure.

Some researchers have expressed skepticism about the therapy, since the infants were also given standard chemotherapy, and the current work does not definitively separate the effects of the two. Still, these early results are promising.