Here for the first time, we present evidence that an HTLV-1 protein, designated HBZ, previously found expressed only in the nucleus, is indeed exclusively localized in the cytoplasm of peripheral blood mononuclear cells of HAM/TSP patients and almost exclusively in the CD4+ T cell compartment without the need that these cells co-express the Treg-associated marker CD25. The gene discussed is HBZ; the disease is tropical spastic paraparesis.