Some studies have shown that people with sickle-cell disease might be more prone to malignant blood cancers. If precancerous cells were among those that were removed, altered and reinfused, the treatment might have somehow given them a boost that allows them to take over the population and become cancerous. “This is a principal, long-term, potential complication that we’re going to have to sort out,” says Walters.

Vertex and CRISPR Therapeutics have proposed to follow trial participants for 15 years after their treatment, to look for long-term impacts of their treatment. In the meantime, it is important that physicians inform their patients of the risks, says Tosin Ola, founder of the advocacy group Sickle Cell Warriors in San Marcos, California. “These are things that I know because I’m a nurse and I do advocacy,” she says. “But a typical mom who is worried for her kid who has sickle cell, she won’t know it. She’ll just sign up.”

I really, really, hope we approach this carefully.