CCR4 and neoplasm: Agents developed for depletion of Treg cells in humans have included daclizumab (Zenapax), a monoclonal antibody against CD25 which is expressed highly on the surface of most Treg cells, denikeukin difitox (Ontak), an IL-2:diphtheria toxin fusion protein that targets Treg cells through their ability to bind IL-2, and mogamulizumab, a depleting monoclonal antibody against CCR4, which is expressed by high frequencies of tumor-infiltrating Treg cells (29).