In this study, we have characterized that a member of the SLAM family of receptors, CD244, was overexpressed on CD8+ T cells of HTLV-I-infected patients compared to NDs, and demonstrated that the upregulation of the adaptor protein, SAP, in CD8+ T cells distinguished patients with HAM/TSP from ACs. The gene discussed is SH2D1A; the disease is tropical spastic paraparesis.