The second principal concern is that the cancer will come back. As a heartrending story in the Washington Post describes, by the time children enter into a CAR-T trial they (as well as the families that support them) have already been through hell. In the case of eight-year old Ava Christianson, the focus of the Post’s piece, the cancer survived even a round of CAR-T treatment, in which Ava’s immune cells were harvested from her, genetically modified, and given back to her. She was in for another, newer generation of the treatment, and it appeared to be working.

“We just need this to work and stay working for her,” her mom, Bethany Christianson, told the Post as she held back tears.

That’s the thing about CAR-T. Powerful as it is, it is such a new therapy that no one knows whether the effects are permanent—and if they aren’t, how long they last. Last year, we followed the story of one patient, Milton Wright III, who at 18 months had been one of the treatment’s longest survivors.

The phrase “a cure for cancer” has been around so long, it’s become cliché—a term invoked figuratively to describe a great achievement, or a panacea. As we approach such a cure, however, the reality is turning out to be far more fragile.

(Read more: The Washington Post, “10 Breakthrough Technologies 2016: Immune Engineering,” “Biotech’s Coming Cancer Cure”)