ISG15 and cancer: In contrast to the abovementioned reports of enhanced ISG15 expression and ISGylation in cancer cell lines and tumor cells, expression profiling and cytogenetic analyses have reported reduced expression of ISGs in immune cells [117], myeloid leukemia cells [118, 119], and cells from colorectal [120, 121], breast [122], prostate [123], and melanoma [124] tumors.