Haplotypes exist because certain polymorphisms tend to be linked to others on the chromosome.
Defining the haplotypes that exist in a patient is important for two reasons. First, the complexity of genetic analysis can be greatly reduced. Fortunately, due to linkage disequilibrium, only a small number of common haplotypes are generally found in the population. This provides a powerful tool to identify correlations that exist between drug response and specific haplotypes. The second, more crucial reason for defining patient haplotypes is that haplotypes are thought to predict the activity of a gene more precisely than genotypes.