after screening for variation with a method such as single strand conformation polymorphism analysis. Types A polymorphism can be any sequence difference. Examples include: Single nucleotide polymorphisms (SNPs) are a single nucleotide changes that happen in the genome in a particular location. The single nucleotide polymorphism is the most common form of genetic variation. Small-scale insertions/deletions (Indels) consist of insertions or deletions of bases in DNA. Polymorphic repetitive elements. Active transposable elements can also cause polymorphism by inserting themselves in new locations. For example, repetitive elements of the Alu and LINE1 families cause polymorphisms in human genome. Microsatellites are repeats of 1-6 base pairs of DNA sequence. Microsatellites are commonly used as a molecular markers especially for identifying the relationship between alleles Clinical significance Lung cancer Polymorphisms have been discovered in multiple XPD exons. XPD refers to "xeroderma pigmentosum group D" and is involved in a DNA repair mechanism used during DNA replication. XPD works by cutting and removing segments of DNA that have been damaged due to things such as cigarette smoking and inhalation of other environmental carcinogens. Asp312Asn and Lys751Gln are the two common polymorphisms of XPD that result in a change in a single amino acid. This variation in Asn and Gln alleles has been related to individuals having a reduced DNA repair efficiency. Several studies have been conducted to see if this diminished capacity to repair DNA is related to an increased risk of lung cancer. These studies examined the XPD gene in lung cancer patients of varying age, gender, race, and pack-years. The studies provided mixed results, from concluding individuals who are homozygous for the Asn allele or homozygous for the Gln allele had an increased risk of developing lung cancer, to finding no statistical significance between smokers who have either allele polymorphism and their susceptibility to lung cancer. Research continues to be conducted to determine the relationship between XPD polymorphisms and lung cancer risk. Asthma Asthma is an inflammatory disease of the lungs and more than 100 loci have been identified as contributing to the development and severity of the condition. By using the traditional linkage analysis, these asthma correlated genes were able to be identified in small quantities using genome-wide association studies (GWAS). There have been a number of studies looking into various polymorphisms of asthma-associated genes and how those polymorphisms interact with the carrier's environment. One example is the gene CD14, which