According to previous reports, this phenomenon of spontaneous PBMC proliferation in patients with HAM is thought to mimic the hyperimmune response observed in HAM, which consists of an increase in and activation of infected cells, followed by a proliferative response of CD8+ T cells that are specific for viral antigens, such as tax and env, which are expressed by infected cells (Itoyama et al., 1988; Ijichi et al., 1989). Here, ERVW-1 is linked to tropical spastic paraparesis.