Therefore, other circulating biomarkers have been explored for NPC diagnosis, such as DNA methylation markers [14], fragments of cytokeratin 18 [15], cathepsin B (CTSB) and D (CTSD) [16], insulin like growth factor 1 (IGF-1) [17], arginase 2 (ARG2) [18], autoantibodies against peroxiredoxin 2 (PRDX2) and PRDX3 [19], and long noncoding RNAs [20]. Here, CTSB is linked to nasopharyngeal carcinoma.