Mouse is the most preferred mammal for studies on gene transfers due to its many favourable features like short oestrous cycle and gestation period, relatively short generation time, production of several offsprings per pregnancy (i.e. litter),convenient in vitro fertilization, successful culture of embryos in vitro, etc. Transgenic mice are being used to test the safety of vaccines (e.g., polio vaccine) before they are used on human beings.