Understanding of linkage disequilibrium (LD) in human populations could 
facilitate the discovery of genes that influence complex human diseases. The 
"HapMap" project is now underway to characterize patterns of LD in the human 
genome. A pilot study showed "haplotype blocks" in 51 regions scattered 
throughout the genome. These intriguing results raise important questions about 
the nature of recombination, and highlight practical issues of marker 
collection, the influence of statistical modelling on apparent block structure, 
and the levels of genotyping necessary for studies of common diseases. Knowledge 
of local disequilibrium patterns may help identify common polymorphisms involved 
in complex disease, but completely new analytical methods and experimental 
designs will be required to identify important rare variants.