Nevertheless, CRISPR-Cas9 has some important limitations as a human genome editor that may make other alternatives more attractive in the long term. The compound’s large size makes it more difficult to deliver into the cell than alternatives like zinc finger nucleases (ZNFs) and TALENs. CRISPR-Cas9 also seems to be more likely to trigger immune reactions.10
Gene-based therapy and enhancement is therefore awaiting technological developments. While TALENS has been used to treat otherwise incurable childhood leukemia and ZNF has been used to treat Hunter’s syndrome in vivo, for the moment, these alternatives to CRISPR-Cas9 are less popular, since they are harder to use. Next-generation genome editors, such as mobile genetic elements, are already being used in the lab, and the future will probably see a combination of these technologies used, depending on the application. The boundary between what is considered therapy and enhancement is already somewhat blurred. Once in vivo genome edits become easy, cheap and mainstream, the line will become even more blurred, as will lines that demarcate prevention from treatment.