Numerous epidemiological studies have shown that the risk of coronary heart disease is strongly influenced by plasma lipid levels, especially HDL and LDL cholesterol, and their specific apolipoprotein constituents. Genetic studies have established significant heritability of these lipid components, and have also identified relatively rare major genes that result in extreme lipid values and increased risk of coronary heart disease. Geneticists have identified a number of segregating polymorphisms at the four major apolipoprotein genomic regions, using a combination of protein and DNA assays. However, in 1988 when the study was initiated, the relationship between these polymorphisms and risk of coronary heart disease had not yet been properly defined.