CD134/OX40 is a receptor of OX40 ligand (OX40L) transactivated by Tax of HTLV-1. OX40 is expressed in activated CD4+ and CD8+ T cells. OX40 on the activated T cells have a role in proliferation of T cells and production of cytokines by binding with OX40L expressed on antigen presenting cells like dendritic cells or activated B cells. In addition, it also be involved in activated T cell invasion to inflamed area through the binding with OX40L.