MDC1 and nasopharyngeal carcinoma: As IR cytotoxicity is mainly due to DNA damage, we explored the correlation between DNA damage repair genes and radioresistance, and found that the mRNA expression of NFBD1, BRCA1, BRCA2, RPA1 and RAD51 was significantly decreased in poorly differentiated NPC cell line CNE2 compared with well differentiated NPC cell lines CNE1 and HNE1 (Fig. 4A).