OX40 is a member of the TNF receptor family that is expressed primarily on activated CD4+ T cells and promotes the development of effector and memory T cells. Higher levels of soluble OX40 was detected in the CSF of HAM/TSP patients with rapid progression, and OX40 was overexpressed in spinal cord infiltrating mononuclear cells in a clinically progressive HAM/TSP patient with a short duration of illness [108]. This evidence concerns the gene CD4 and tropical spastic paraparesis.