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Prevalence of many complex human diseases such as asthma, cardiovascular disease, and diabetes has risen greatly over the past two decades in developed countries.1,2 During the same period, the genetic causes of such diseases have been increasingly emphasised as a means to better understand their pathogenesis, with the ultimate goal of improvement of preventive strategies, diagnostic tools, and treatment.3–8 Considerable effort is being expended in attempts to detect genetic loci contributing to complex diseases.9 Association and linkage studies comprise the two dominant strategies: association studies aim to find disease-predisposing alleles at the population level; and linkage studies focus on familial segregation. Although both strategies have compelling strengths, association analyses are more widely done and likely to spread even further in the future, especially in the pharmacogenetics domain.5 Technical developments in molecular genetics facilitate these studies, as does use of gene-specific variants derived from the human genome sequencing project.10 Furthermore, extensive catalogues of anonymous DNA sequence variants across the human genome are being compiled.11,12 Some large-scale, population-based human samples have been, or are expected to be, gathered (eg, EPIC,13 ISIS,14 Million Women Study,15 MRC/Wellcome Trust Biobank UK),16–19 and use of DNA variants in drug development is expanding.4 Coupling of high-throughput molecular technology, many genetic variants, and population-based samples offers unique opportunities for understanding the cause of common diseases. Genetic variants—or polymorphisms—arise from new mutations. The simplest type of polymorphism is a single base mutation, which substitutes one nucleotide fo