A genetic polymorphism may act by affecting the regulation of expression of the candidate gene or by leading to a changed composition and function of the protein that it encodes. The selection of SNPs in or around the candidate gene region should take account of in-depth characterization of the functional elements identified therein and how their localization in relation to the gene might affect its function. Nevertheless, it is important to note that functional elements both within and outside boundaries of a gene may well be unknown at the time of study, and our knowledge of functionality is expanding continuously. Regulatory regions such as promoters and enhancers are essential for controlling the extent to which the gene is transcribed. Polymorphisms in coding exons and regulatory regions are less frequent than variations in a part of a sequence with no clear functional role such as intergenic regions and introns. This is explained by the fact that altering the function of the fine-tuned biological system will likely have a detrimental effect on the functioning and survival of the individual, the result of
