Abstract: This invention features our discovery on usages of Methylenetetrahydrofolate Reductase (MTHFR) gene polymorphisms in predicting homocysteine (Hcy) level and/or incidence and prognosis of diseases associated with increased Hcy level in a subject, as well as predicting treatment effects of medicines in the category of Angiotension Converting Enzyme Inhibihor (ACEI) with and without combination with B Vitamins. This invention also features our discovery on laboratory and analytical methods that are essential to the above described usages of MTHFR gene polymorphisms. In addition, this invention features a kit that has translated the above discoveries into a practical and reliable tool that can be applied to accomplish the above described usages of MTHFR gene polymorphisms. This invention represents an important step in realizing personalized medicine, with the goal to tailor diagnosis, prevention and treatment strategy to meet individual needs.