Aging is characterized by progressive and broadly predictable changes associated with increased susceptibility to many diseases. Aging is the major risk factor for the development of chronic diseases such as cardiovascular disease, cancer, diabetes, and dementia. Therefore, drugs that slow the aging process may help extend both lifespan and health span (the length of time that people are healthy). Aging is not a uniform process. Rather, organs in the same person age at different rates influenced by multiple factors, including genetic makeup, lifestyle choices, and environmental exposures. As an example, mutations accumulate in stem cells with aging but are different depending on the organ. Epigenetic DNA modifications with age are also highly tissue specific. A twin study found that genetics accounted for approximately 25 percent of the variation in longevity among twins and environmental factors accounted for approximately 50 percent. However, with greater longevity (to age 90 percent), genetic influences became more important. For example, Metformin could become the first authorized anti-aging drug with the ability to not just prolong life, but to prolong a healthy life. Then after metformin, other anti-aging drugs could follow.