The zebrafish (Danio rerio) is an increasingly popular model for research on vertebrate biology and human disease, thanks in part to its short generation time and transparent, externally fertilized eggs, which facilitate studies of embryonic development. An array of molecular tools has been applied to zebrafish studies, including various genome modification techniques. For example, zinc finger nucleases (ZFNs) have been used to create DNA mutations at specific locations, and gene regulation molecules called morpholinos have been used to manipulate gene behavior. These methods have several limitations: they are relatively inefficient and slow, they can target only specific sequences and their effects may be temporary. According to Stephen C. Ekker (Mayo Clinic, Rochester, MN) and his collaborators, the inability to modify the zebrafish genome in a targeted fashion has hindered zebrafish research.Lab Anim. (NY) 41, 300 (2012).