Understanding the molecular basis of quantitative variation has the potential of explaining numerous phenotypes, including common human diseases such as asthma, atherosclerosis, diabetes, hypertension, and obesity. The goal of QTL (Quantitative Trait Loci) mapping is to identify and characterize the DNA sequences responsible for these conditions. Although it has been very difficult to conduct linkage studies in humans, QTL have been mapped in inbred species: rodents, insects and plants. Despite this progress, finding the genes underlying quantitative traits remains a challenge.