CH12F3 is capable of cytokine-induced Ig heavy chain CSR. Key enzymes involved in DNA recombination and joining double strand breaks (DSBs), such as DNA Ligase 1, Ligase 3, Ligase 4 and Apyrimidinic Endonuclease APE1, have been knocked out in these CH12F3 cell lines, providing ideal genetic materials to investigate the roles of these proteins in regulating the proficient CSR event in mature B lymphocyte cell lines. Since enzymes involved in DNA metabolism play essential roles in embryogenesis, knockout of these proteins often resulted in lethality in mouse embryonic stem cells. However, knockout of these proteins in mature B cells are compatible with cell viability