Transgenic Animals - A transgenic animal contains in its genome, a gene or genes introduced by one or the other technique of transfection. The gene introduced by transfection is called transgene. In animals, transfection specifies the introduction of a DNA segment, either naked or integrated into a vector, into an animal cell.

It may be pointed out that the same phenomenon is known as transformation in all other organisms. However, in case of animals, transformation has long been used to describe the change of normal, i.e., nontumourous, cells in culture to tumour like cells. Therefore, it became necessary to use the term transfection to avoid confusion.