CRISPR gene editing has been one of the important advances of the last decade, in biotechnology and increasingly in medicine. First applied to human cells in 2013, and honored with the 2020 Nobel Prize in Physiology or Medicine, its meteoric rise can make CRISPR look like the molecular equivalent of a miracle healer. But in the research and clinical trenches, CRISPR-based approaches, like any others, need to find applications where their desired effects outweigh their side effects. And finding those applications necessitates ways to identify off-target effects.