CD163 is considered a marker for M2-polarized macrophages, hence, the presence of numerous CD163+cells supports the hypothesis of an activation of immunosuppression and immunoescape mechanisms within the tumor microenvironment also in canine melanomas, as we previously hypothesized by testing the expression of IDO, Foxp3, and CTLA-4 (41, 54). The gene discussed is IDO1; the disease is melanoma.