Zinc-finger nucleases (ZFNs), used for targeted genomic modification, can cross the cell membrane as proteins. The findings, published this week in Nature Methods, should enable simpler and safer delivery of these biological tools to cells.

Designer nucleases, such as ZFNs, can be used to make targeted changes in the genomes of many species and are generating excitement as tools for both research and gene therapy. The nucleases are typically delivered to the inside of cells as DNA or RNA, where the functional proteins are then generated.

Carlos Barbas and colleagues report that ZFN proteins themselves can cross the cell membrane. They do so-as observed in several mammalian cell types-at high enough levels to generate targeted genomic changes as efficiently as ZFNs delivered as DNA. Although cell membrane permeation is a property that has been reported for a small set of proteins and peptides, it was not previously known to be a feature of ZFNs.

Nucleases delivered as proteins are present in the target cells transiently and generate measurably fewer off-target genomic changes than nucleases delivered as DNA. Protein delivery of these tools avoids the risk of insertional mutagenesis associated with delivery by viral vectors as well as the toxicity associated with the cellular response to exogenous RNA.