To create a more robust source of genetically-defined human cell lines, a revolutionary technology was needed. To realize this aim, Horizon applied a seminal discovery by Professor David Russell that certain Adeno-Associated Virus (AAV) homologous recombination vectors (comprising single-stranded homologous DNA and not requiring double-strand breaks) were more than 1,000 times more efficient at gene-targeting in human somatic cell-types than plasmid-based methods. By initiating homologous recombination, any sequence alterations contained within the homologous DNA are recombined with high precision into the target gene.   
   
The use of rAAV vectors in gene-targeting has since been refined by Horizon’s collaborators and co-founders into a robust and refined gene-editing platform, and this technology now allows (via a 10 fold further increase in gene targeting efficiency) the precise and specific ability to edit-on-demand the genome of any mammalian cell down to single base-pair resolution and without sequence error.