We are developing a useful antibody production technology that overcomes the shortcomings of conventional technologies by mimicking the in vivo antibody production mechanism using a cultured B cell line with the ability to mutate antibody genes. The chicken B cell line DT40 not only produces antibodies and has the ability to mutate antibody genes, but also has high homologous recombination efficiency without the use of special techniques, making it an ideal cell line for developing antibody generation technology by manipulating cell functions at the genetic level. We have developed an in vitro technology for antibody generation based on a cell line in which the mutation machinery of DT40 cells can be turned on and off. We are now working on the application of this technology to the generation of human antibodies, unconventional antibodies, or modification of proteins of interest, by modifying antibody-related genes in DT40 cells.