Genotyping costs have come down substantially, and publicly available datasets, such as dbSNP and the International HapMap, have provided deeper insight into genetic variation in genes. This knowledge has led to more comprehensive studies that systematically examine the association of all common variation in a gene of interest by means of carefully selected tagSNPs and their haplotypes under the assumption that a causal variant would be in high linkage disequilibrium with one of the tagSNPs or at least captured by the haplotypes [22]. Perhaps the most important finding in hunting obesity gene occurred in 1994, when Zhang and coworkers demonstrated for the first time that an adipose-derived hormone, leptin, plays a key role in regulating intake and energy expenditure, including appetite and metabolism [23, 24]. This discovery represented a huge step forward in the study of obesity. Mutations in leptin and leptin receptor genes have been associated with mild to extreme obesity phenotype in human population [25–28].
