While the majority of HTLV-1-infected individuals remain asymptomatic, after a long period of clinical latency a low percentage of patients develop either adult T-cell leukemia/lymphoma (ATLL), a disease characterized by malignant proliferation of CD4+ T-lymphocytes, or HTLV-1-associated myelopathy/ tropical spastic paraparesis (HAM/TSP), a neurodegenerative condition of possible auto-immune nature [4,5,6,7,8,9,10,11,12,13]. Here, CD4 is linked to tropical spastic paraparesis.