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The targeted transgenic mouse approach uses gene targeting by means of homologous recombination in embryonic stem (ES) cells. The transgene cDNA is knocked into a ‘safe habor’ locus in the mouse genome to generate transgenic mice with a predictable expression pattern without disturbing endogenous gene activity. Safe harbor loci have been utilised in the mouse to investigate gene function, generate humanized models and create tissue-specific reporter and Cre recombinase expressing lines.

Knockout mice are animal models that have been genetically modified to delete or inactivate a specific gene. The term ‘knockout’ refers to knocking out the functionality of the target gene. These mouse models allow researchers to gain insight into how the loss of the gene affects physiology, behavior and disease development. Observing the characteristics of a knockout phenotype can also help researchers understand human gene expression and genetic diseases.

Knock-in mice have an insertion in a specific locus in order to produce genetically modified mice for customised research purposes. Reporter genes, human genes, related genes from the same organism, or point mutations can be introduced into the protein-coding region of a gene to produce knock-in mice. The difference between the knock-in and knockout approach is that knock-in mouse models are utilised to drive expression of an exogenous transgene or gene mutation, whereas knockout mice aim to disrupt the expression of a specific wild-type gene.

Humanized mice broadly refers to mice expressing human genes via transgenesis to gain relevant insights in the in vivo context for the understanding of human-specific physiology and pathology. Whilst many basic biological insights have been obtained from mouse studies, there are limitations to mouse models when investigating human biology. There are components of mouse biology systems that are inconsistent with their human counterparts, particularly the immune system. Humanized mouse models have begun to fill this gap and have become important pre-clinical tools for biomedical research. The use of humanized mice may facilitate the application of customised treatment in the medical industry.