 Using ZFNs to modify endogenous genes has traditionally been a difficult task due mainly to the challenge of generating zinc finger domains that target the desired sequence with sufficient specificity.Improved methods of engineering zinc finger domains and the availability of ZFNs from a commercial supplier now put this technology in the hands of increasing numbers of researchers.Several groups are also developing other types of engineered nucleases including engineered homing endonucleases
 and nucleases based on engineered TAL effectors.