Mechanisms for targeted, purposeful mutation revealed in an APOBEC–DNA complex

Targeted deamination of cytosine bases in DNA by AID/APOBEC-family enzymes is critical for proper immune function, but it also poses risks to genomic integrity. New structures reported by Harris, Aihara and colleagues offer the first glimpses into the enzyme–DNA complex and reveal both expected and unexpected insights into the DNA-binding mode involved in targeting purposeful mutation.