A previous study has shown that checkpoint inhibitors upregulate the expression of CCL17 and CCL22 in tumors, leading to Treg migration into the microenvironment and resistance to treatment, and that blockade of CCR4 augmented the antitumor effects of checkpoint inhibitors.9 In this study, we showed that mogamulizumab is effective in dogs with prostate cancer, and gene expression signatures in some human patients with prostate cancer was similar to canines, indicating that patients with prostate cancer with canine-like subtypes might benefit from mogamulizumab treatment. Here, CCL17 is linked to prostate cancer.