<qa>  What is the main limitation of genetic approaches to study of aging?Under the assumption that aging is caused by dysfunction of specific biological mechanisms, it is reasonable to hypothesize that slowing aging should delay the onset of chronic diseases that typically affect older persons and improve their longevity and quality of life. Indeed, there is emerging evidence that factors associated with premature mortality are also involved in multiple pathologic conditions typical of aging. Scientists have suggested that these factors are genetic in nature and involve variations in the genetic code sequence. Contrary to this view, genetic approaches to study of aging have had limited success and it has been argued that the study of aging and longevity requires a more comprehensive analysis that includes non-genetic biomarkers such as epigenetic, gene expression and protein biomarkers. This is because these non-genetic biomarkers reflect the interaction between genetic variation and environmentalbehavioral factors. A major limitation in this approach to date is that most studies of epigenetic, gene expression and protein biomarkers rely on blood specimens, which may not recapitulate the biology of other tissues. In addition, although all cell types from the same person have exactly the same genetic code; information on epigenetic modifications, RNAm, and protein expression likely differ across cell types and at different points in time. Thus, global measures of these biomarkers in specific cell types can be affected by percentages of these cell types in the blood, and it is well know that such percentages change with aging and chronic diseases. 