**5. Conclusion**

DNA repair genes play a pivotal role in the maintenance of genome integrity. Alterations of various DNA repair genes, either in gene sequence/structure or in gene expression, are frequently found in most of human malignancies. Since DNA repair activity is able to modulate cellular response to DNA-damaging anticancer drugs, alterations of DNA repair genes may be involved in the development of resistance to chemotherapy and radiotherapy. In addition, DNA repair activity plays an important role in preventing the mutagenicity and cytotoxicity induced by numerous environmental carcinogens and toxicants. Cells with reduced DNA repair activity may thus be prone to pathological transformation. Therefore, examining the DNA repair activity of a cell can help us to understand the probability of cellular tumorigenicity associated with exposure of environmental carcinogens and is able to assess the responses of various regimens of anticancer treatment. HCR assay is an easy and fast functional assay that can be applied to investigate several DNA repair pathways and is one of the most useful methods for evaluating cellular DNA repair activity *in vivo*.
