WHEN THE first draft of the DNA sequence that makes up the human genome was unveiled in 2000, America’s president at the time, Bill Clinton, announced that humankind was “learning the language with which God created life”. His assessment was a little quick off the mark. For one thing, the full sequence would not be completed until 2022. For another, whereas scientists can use sequencing tools to read DNA, and CRISPR technology to make small edits, actually writing the genomic language has proved trickier.