Total RNA was extracted from BMCs with TRIzol Reagent (Invitrogen, Carlsbad, CA, USA) according to the procedure supplied by the manufacturer. One hundred nanograms of each RNA was used for reverse transcription that was performed with RevertAid First Strand cDNA Synthesis Kit (Fermentas, Lithuania) according to the manufacturer's protocol. The resulting cDNA was diluted in DNase, RNase-free dH2O (Invitrogen, Carlsbad, CA, USA). Next, 1 μl of cDNA corresponding to 0.5 ng of total RNA was used as a template in real-time PCR. The reaction was performed in LightCycler 480 Instrument II (Roche, Mannheim, Germany) with LightCycler 480 Sybr Green I Master kit (Roche, Mannheim, Germany) and with XPD 50CATTTCCTGGGCTTCCTGAG30 sense and 50GCTCTCCTGCACCACATGCT30 antisense primers, as well as with sense 50CAGCCTGGATAGCAACGTACA30 and 50TTCTACAATGAGCTGCGDNA repair capacity is associated with longevity. Our findings support the notion that DNA repair capacity decreases with age and that XPD plays an important role in maintaining genomic stability. The decreased amount of XPD mRNA and changes in the methylation pattern of the XPD promoter may contribute to the decreased DNA repair capacity in older individuals. Our study has some limitations, including a small sample size for promoter methylation analysis and the absence of an intermediate age group for polymorphism analysis. However, our results are consistent with in vitro and clinical findings and suggest that the decreased frequencies of function-impairing AA and cancer-prone CC genotypes of the XPD Lys751Gln polymorphism promote longevity thanks to efficient DNA repair and lower risk of developing life-threatening diseases. Further studies are needed to confirm these findings and to explore other regulatory mechanisms that may control XPD mRNA levels.Protein function is associated with aging. Such XPD-related changes support the notion that aging is associated with altered DNA repair and transcription. However, studies have shown that XPD polymorphisms have no effect on the DNA repair efficiency of XPD (Lunn et al. 2000; Hemminki et al. 2001; Laine et al. 2007). In addition, results regarding the role of this polymorphism in carcinogenesis are quite inconsistent, showing an association between the A variant genotype (encoding Asn) with either decreased (Lovatt et al. 2005; He et al. 2008) or, more commonly, increased (Hu et al. 2004; Wang et al. 2008; Gangwar et al. 2009) risk of cancer development. Data regarding the Arg156Arg polymorphism are sparse and also inconsistent: the presence of the A allele was correlated with both increased (Harth et al. 2008) and decreased (Lovatt et al. 2005) cancer risk. It is likely that these polymorphisms do not play a universal role in cancer pathogenesis. Our results show that... 

Acknowledgments: This work was supported by the Polish Ministry of Education and Science grant PBZ-MEiN-9/2/2006 (K143/P01/2007/1).

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