Conversion of the genetic background of transgenic mice by in vitro fertilization

We attempted to convert the genetic background of transgenic (Tg) mice in a short period of time by applying in vitro fertilization techniques. Tg mice were obtained by injecting human interleukin-2 (hIL-2) gene into the fertilized eggs of the C57BL/6N strain. These Tg mice were back-crossed 8 times to the inbred C3H/HeN strain using the hIL-2 gene as the genetic selection marker. In order to shorten the length of the back-crossing time, in vitro fertilization was performed with eggs collected from immature Tg females and spermatozoa from mature C3H/HeN males, and successfully fertilized eggs (2-cell stage) were transferred to pseudopregnant recipients to obtain the offspring of next generation. When no offspring were obtained through the procedures using immature Tg females, in vitro fertilization was performed with mature Tg males and mature C3H/HeN females to continue successive back-crosses. With this method, it was possible to perform 8 successive back-crosses in 18 months.