In vivo and irrespective of the clinical status (i.e., in asymptomatic carriers (ACs), ATLL or HAM/TSP patients) the HTLV-1 provirus is predominantly detected in CD4+ T lymphocytes [38], even if infection of CD8+ T lymphocytes [39–41], and to a lesser extent of B lymphocytes has also been documented [42]. This evidence concerns the gene CD4 and tropical spastic paraparesis.