Since November, many researchers around the world have called the work done by the Chinese team irresponsible. That's mainly because, from a research ethics perspective, scientists still don't know enough about the CRISPR technology to apply it to humans just yet. There are still unresolved issues like off-target effects, in which the CRISPR technology edits genes that look like the ones they are meant to cut, but aren't, and mosaicism, in which only part of the entire genome gets edited. Both of these deficiencies, when they accumulate, will have effects that are unpredictable to scientists. The stakes are elevated even higher when you edit germ-line cells, as the Chinese researchers did. These genetic tweaks remain for every generation that follows; so those twin babies, if they had children, would pass on the genetic alteration for CCR5.