Gene targeting in the mouse is carried out by culturing the embryonic stem cells (ES). These embryonic stem cells are capable to differentiate into any cell type in the body. In a particular tissue or in the organ, the stem cells are undifferentiated and divide irregularly to develop a progeny stem cell.
To produce a mouse that is homozygous for a rat growth-hormone (RGH) transgene, first inject the rat growth-hormone into the fertilized eggs of mice and insert into surrogate mother mice. Observe for the resulted offspring, and test the offspring to find out if any of them are transgenic for rat growth-hormone, as the RGH will be present at birth in a dominant manner. Large and transgenic offspring will be heterozygous. The siblings that are transgenic will require to be reproducing to each other in turn to develop mice that are homozygous for the rat growth-hormone transgene.