Although GWA studies are no doubt a powerful tool, they also have limitations. Currently, genetic variants included in SNP chips for GWA studies are focused on common variants; rare mutations are ignored in general. New chips may be more inclusive. Also, some genes of interest in asthma research are barely covered by SNP chips. For example, IL4, an asthma candidate gene for which positive association with asthma had been reported in 17 independent populations previously, is not covered with a single SNP in currently used SNP chips (M. Kabesch, unpublished data). Here, imputation may prove helpful. With this technique the allelic states at polymorphic sites not genotyped can be deduced from the surrounding SNP information based on linkage disequilibrium and likelihood estimates. To improve the accuracy of this technique and our knowledge of linkage disequilibrium in the genome, a project sequencing 1,000 human genomes is currently ongoing.