The researchers created the DNA-snipping enzyme called Brec1 using directed evolution, an engineering technique that mimics proteins’ natural evolution process. They programmed the enzyme to cut DNA on either side of a sequence characteristic of HIV—a difficult task since the DNA of organisms and of the virus itself mutates often. Still, the researchers identified a well-conserved sequence, then they tested how reliably the enzyme could snip out that sequence in cells taken from HIV-positive patients, in bacteria, and in mice infected with the human form of HIV. After a number of tweaks, Brec1 would cut only that sequence of DNA, patching up the cell’s genetic code once the HIV sequence was cleaved out. After 21 weeks, the cells treated with Brec1 showed no signs of HIV.