The biologist from Oregon Health & Science University shocked people in 2017 when he repaired a genetic mutation causing heart disease in dozens of human embryos. (The embryos were destroyed as per ethical requirements of the experiment.) He used a controversial gene-editing technique called CRISPR that has yet to be proven safe and effective for treating human disease, and critics questioned his results. But this year, Mitalipov defended his findings after re-analyzing the DNA from the embryonic cells, and other groups have reported similar results using CRISPR to repair the mutation in mouse embryos. Mitalipov says he also tested the technique with inherited mutations that cause other diseases, with similar repair success. He sees his studies as the first step toward IVF gene therapy, in which researchers can repair inherited genetic diseases in IVF embryos before they are implanted in the womb. —Alice Park
Robbie McClaran—Redux for TIME