CCR4 is an important molecule in HTLV-1 infection, not only because its mutation significantly contributes to the development of ATL as described above, but also because it serves as a marker for ATL and HAM/TSP (44, 45), and is targeted by the monoclonal antibody mogamulizumab, a clinically approved drug for the treatment of ATL and HAM/TSP (46, 47). Here, CCR4 is linked to tropical spastic paraparesis.