Gene targeting was developed in mammalian cells in the 1980s, with diverse applications possible as a result of being able to make specific sequence changes at a target genomic site, such as the study of gene function or human disease, particularly in mice models. Indeed, gene targeting has been widely used to study human genetic diseases by removing ("knocking out"), or adding ("knocking in"), specific mutations of interest. Previously used to engineer rat cell models, advances in gene targeting technologies enable a new wave of isogenic human disease models. These models are the most accurate in vitro models available to researchers and facilitate the development of personalized drugs and diagnostics, particularly in oncology. Gene targeting has also been investigated for gene therapy to correct disease-causing mutations. However the low efficiency of delivery of the gene-targeting machinery into cells has hindered this, with research conducted into viral vectors for gene targeting to try and address these challenges.