A complex coordination of diverse cells maintains life in higher organisms. Any disturbance in this coordination can lead to various diseases. Therefore, individual-level analysis using appropriate animal models is essential to understand gene function and associated life phenomena in higher organisms. Our laboratory is developing technologies that allow for the rapid generation of diverse genetically engineered mice using genome editing technology applied to mouse-fertilized eggs or embryonic stem cells. Another research interest is to uncover hidden characteristics of germ cells. Germ cells are unique cells that play a crucial role in transmitting genetic information from one generation to the next. Abnormal germ cell development is linked to infertility, as well as the development of germ cell tumors or testicular tumors. Using mouse models, our laboratory investigates the mechanisms that regulate germ cell development and spermatogenesis. By leveraging this technology, we aim to deepen our understanding of these processes and ultimately contribute to developing therapies to address these and other related conditions.