ISG15 and cancer: Interestingly, the protein expression level of ISG15 showed a subtle yet significant positive correlation with the protein expression level of SIRT1 in the complete set of cancer types (R = 0.15, P = 0.0042) as well as in non-small cell lung carcinoma (NSCLC) (R = 0.26, P = 0.0441) (Fig. 2d).