Mice have long served as models for studying human biology and pathology because of their phylogenetic and physiological similarity with humans. They are also easy to maintain and breed in the laboratory, and hence, many inbred strains are now available for research. Studies on mice have contributed immeasurably to our understanding of cancer biology.

The development of transgenic, knockout, and knock-in mice has led to an exponential increase in their use as model organisms in research, including cancer research. A gene-of-interest can be added or deleted from a mouse’s genome using genetic engineering techniques, and if it causes tumor initiation in mice, it is highly likely to be a cancer-critical gene in humans. In addition, mouse models such as reporter mice can also be used to study the crucial stages of tumor initiation and their progression to aggressive cancers. In such mice, a reporter gene with luminescence or fluorescence tags is inserted into the cells under study to monitor their growth and proliferation.

Pre-clinical studies in mouse models serve as a perfect step between in vitro studies on cell cultures and clinical studies in humans. They can be used for investigating in vivo pharmacokinetics, toxicity, and anti-tumor efficacy of numerous chemotherapeutic agents before any of the drugs go to clinical trials. However, since mouse tumor cells or immune responses may not exactly represent the tumorigenic process in humans, mouse models have been developed that can closely mimic the tumor progression and immune responses in humans, for example, human tumor xenografts in immunocompromised mice or humanized mouse models.

In vivo testing in animals, including mice, can often raise ethical concerns. While it is true that experimentation in animals causes suffering, the benefits of animal experimentation in drug discovery are undeniable. Therefore, animal studies should be strictly avoided wherever alternative testing methods are available. In addition, experiments on animals should only be conducted after in vitro studies have been successfully performed on suitable cell lines.