A humanized anti-human CCR4, mogamulizumab, is commercially available for the treatment of CCR4+ adult T-cell leukemia/lymphoma.6 We previously confirmed that mogamulizumab crossreacts with canine CCR4 and depletes Tregs in dogs.21 To assess the clinical efficacy of the anti-CCR4 treatment in canine prostate cancer, we compared 23 dogs that received mogamulizumab and piroxicam with the control arm of 23 age-matched, sex-matched, and tumor stage-matched dogs that received piroxicam alone (online supplemental table S4). Here, CCR4 is linked to prostate carcinoma.