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.