**8. XPD (ERCC2)**

Xeroderma Pigmentosum group D (XPD) also known as ERCC2 (Excision Repair Cross Completing group 2) gene encodes a helicase, a major DNA repair protein, which is involved in transcription-coupled NER and in the removal of a variety of structurally unrelatedas DNA lesions (Lehmann, 2001) including those induced by tobacco carcinogens (Leadon & Cooper, 1993), (Tang et al., 2002). The normal functioning XPD protein plays an essential role in NER and participates in the unwinding of DNA at the site of deleterious DNA lesions (Hoeijmakers et al., 1996). Several studies have reported association between A751C variant of XPD and increased risk of lung cancer (Hou et al., 2002; Spitz et al 2003; Ramachandran et al., 2006). Hou et al., 2003 reported a marginally increased risk for those carrying heterozygous A>C transversions, compared to those with wildtype homozygous, indicating that heterozygosity also carry the risk. In a Northeastern Chinese population, XPD 751 AC heterozygous genotype carriers were at 2.7 fold higher risk of lung cancer than carrier of AA genotype (Yin et al., 2006). A significant association of XPD variants in modulating NSCLC risk was reported by Zienolddiny et al (2006) in Norwegian lung cancer population. So also, in an Indian population, Sreeja et al., (2008) also reported significant association of XPD heterozygous variants in modulating Non small cell lung cancer risk. SNPs in genes involved in nucleotide excision repair (ERCC1, XPD, XPC, XPA, XPF and XPG) and mismatch repair genes (MLH1 and MSH2) in 577 colorectal cancer cases and 307 case-affected sibling controls were examined by Joshi et al., (2009). Their results showed that consumption of red meat, heavily brown on the outside or inside, increased colorectal cancer risk only among subjects with XPD codon 751 Lys/Lys or XPD codon 312 Asp/Asp genotypes.
