CYP2D6 and nasopharyngeal carcinoma: Moreover, PCR analysis revealed that the expression of CYP1A1, CYP2C8, CYP2D6, DHFR, GSTP1, and SULT1E1 genes, that are associated with drug catabolism, were significantly higher in radioresistant NPC cells compared to their parental cells (Figure 5C).