A Simple and Efficient Procedure for Developing Mouse Germline Stem Cell Lines with Gene Knock‐in via CRISPR/Cas9 Technology

Cultured mammalian spermatogonial stem cells (SSCs), also known as germline stem cells (GSCs), hold great promise for applications such as fertility preservation, gene therapy, and animal breeding, particularly in conjunction with accurate gene editing. Although the in vitro development of mouse GSC (mGSC) lines, and gene‐targeting procedures for such lines, were initially established about two decades ago, it remains challenging for beginners to efficiently accomplish these tasks, partly because mGSCs proliferate more slowly and are more resistant to lipid‐mediated gene transfection than pluripotent stem cells (PSCs). Meanwhile, methods for mGSC culture and gene editing have been evolving constantly to become simpler and more efficient. Here, we describe how to develop mGSC lines from small mouse testis samples and how to carry out gene knock‐in in these cells using CRISPR/Cas9 technology, detailing three basic protocols that constitute a streamlined procedure. Using these simple and efficient procedures, site‐specific knock‐in mGSC lines can be obtained in 3 months. We hope that these protocols will help researchers use genetically modified GSCs to explore scientific questions of interest and to accumulate experience for application to GSC research in other mammalian species.