 The realization that the structure of genetic variation in the genome can be understood in terms of haplotypes was important for at least two reasons. First it opened the door for a relatively easy and efficient analysis of (single nucleotide) genetic variation in populations: the clustering of SNPs meant that in principle only one or a few of the SNPs in each cluster (so-called ‘tag SNPs’) would have to be tested to verify the presence of the cluster of variants as a whole. This made the analysis of genetic variation at the level of whole genomes from a large number of subjects feasible at a time when whole-genome sequencing was still too expensive for such a task (HapMap 2003). The development of a haplotype map was therefore a crucial step to enable what are now called ‘genome-wide association studies’ (GWAS) (see  Section 3.1.2).