Our conclusions about the performance of genetic testing for type 2 diabetes are confined to the use of single common alleles at each locus. Other common risk alleles are likely to exist at the same genetic loci (including the causal variants), which could provide additional information relevant to prediction. Our conclusions are also not transferrable to other common diseases. For example, genetic variants underlying the susceptibility to age related macular degeneration have been identified, at least one of which is both common and large in its effect on risk.48 We previously examined the predictive utility of a common single nucleotide polymorphism associated with the risk of coronary heart disease at the 9p23.1 chromosomal locus (rs10757274) when added to a risk function that included variables incorporated in the Framingham coronary heart disease risk equation.49 Although this genotype added minimally to the ability of the Framingham risk score to discriminate future events, improving the area under the receiver operating characteristics curve by only 3%, it did significantly improve reclassification of risk of coronary heart disease, albeit modestly. Moreover, for some disorders, including
