Estimating the cytotoxic T lymphocyte (CTL) level of each sample using the average expression of five marker genes (CD8A, CD8B, GZMA, GZMB and PRF1) (Rooney et al., 2015), we showed that the CTL levels boost up at the early stage but slightly drop at the late stage during NPC progression (Figure 4C). The gene discussed is PRF1; the disease is nasopharyngeal carcinoma.