In the last several years,genetic disease research efforts have increasingly turned from the study of single-gene,Mendelian disorders to the search for DNA sequence variants associated with increased risks for the development of common,or complex,diseases caused by the interaction of hereditary and environmental influences.Because multiple genetic variants,each of which confer relatively low increases in disease risk are present,it is expected that genetic association studies aimed at discovering novel risk alleles will require the testing of DNA samples from hundreds of subjects with thousands of single nucleotide polymorphisms (SNPs).In order to increase the likelihood of success of such studies,a considerable amount of attention will need to be paid to the experimental design of genetic association studies.