1. He didn’t address an unmet medical need.

He focused on a gene called CCR5, which the HIV virus uses as a doorway for infiltrating human cells. To lock the virus out, several scientists have tried extracting the immune cells of HIV patients and deactivating CCR5 using gene-editing techniques before injecting the cells back into the body. Although Nana and Lulu’s father is HIV-positive, neither of the infants actually had HIV. As I’ve written before, He’s team deactivated a perfectly normal gene in an attempt to reduce the risk of a disease that neither child had—and one that can be controlled through safe-sex education or antiviral drugs. Even if you wanted to block CCR5 specifically, there are drugs out there that could do the job, many of which have been repeatedly tested in clinical trials. The rationale for using a method as extreme and untested as gene editing doesn’t hold up.

Read: You may already be immune to CRISPR

Deactivating CCR5 doesn’t confer complete immunity to HIV, either, since some strains of the virus can enter cells via a different protein. And although people with natural deficiencies in the gene appear healthy, they might be more susceptible to West Nile virus, and more likely to die when they catch influenza. Essentially, He gave Nana and Lulu resistance to a virus that they could have avoided in myriad other ways, and may have opened them up to other dangers.

2. The actual editing wasn’t executed well.

He’s data haven’t been published or peer reviewed, so many of the details of his experiment are unclear. But based on the slides that he presented at the Hong Kong summit, other scientists have denounced the work for being amateurish.

For example, it appears that He only managed to edit half of Lulu’s CCR5 genes; the rest are normal. That could either be because every cell in her body has one normal copy of CCR5 and one edited one (she’s heterozygous) or because half of her cells carry two edited genes and half carry two normal ones (she’s mosaic). If it’s the former, she would not be resistant to HIV. If it’s the latter, it depends on whether her immune cells specifically carry the edits. The same might apply to Nana, who, based on the slides, seems to also have normal copies of CCR5 somewhere.

What’s more, the edited cells don’t seem to have been edited in the right way. He planned to delete a small section of the CCR5 gene, mimicking a naturally occurring mutation called delta 32 that’s found in about 10 percent of Europeans. But according to Sean Ryder, a biochemist from the University of Massachusetts Medical School, He’s slides show no sign of delta 32 in either girl. Instead, Lulu has an entirely different CCR5 mutation, and Nana has two. These mutations are in roughly the same part of the gene as delta 32, but “it’s a fairly outrageous assumption that any change to this region would lead to some benefit,” Ryder says. “He made new mutations, and there’s no reason to think that they’d be protective—or even that they’d be safe.”