Deoxyribonucleic acid (DNA) is the cornerstone of all multicellular organisms on the planet, including humans. Defects in DNA sequences can cause the dyshomeostasis of an individual, in turn leading to the onset of any number of diseases. The development of genetic engineering technology in recent years has enabled us to delete specific genes, cause over-expression, or insert dysfunctional genes into animals (knock-out mice, transgenic mice, and knock-in mice, respectively), helping us to better understand biological functions and pathologies, discover drug targets, and evaluate efficacy and feasibility in candidate drugs. Extensive analysis of biological functions is now also possible using “conditional knockout” technology, which allows us to achieve temporal–-spatial control of gene deletion and expression in animals (specifically, conditional knockout mice). Our team continues to actively introduce these and other cutting-edge technologies to ethically promote drug discovery research using genetically modified animals.