Genetically engineered animal models have been used since 1973 as a tool for genetic research. Some of the areas of research have focused on the pathogenesis of different diseases, as well as their underlying mechanisms. The major type of animal models that have been used for this purpose is the "Knock-Out Mice". Knock-out mice are mice phenotypes resulting from inactivation of molecules (genes) responsible for the control of specific characteristics. Suc h modification usually leads to novel insights into the action of the deactivated gene action. This review will focus on nine different articles in which knock-out mice have been demonstrated to be useful in the study of treatment mechanisms for cancer, obesity, heart disease, diabetes, Arthritis, substance abuse, anxiety, ageing and Parkinson's disease.