Although the abundance of interferon-γ (IFN-γ) and tumor necrosis factor-α(TNF-α) producing cells in the HTLV-1 tax-expressing cell population in PBMC of HAM/TSP patients was observed in a comparative study of intracellular cytokine expression levels in HAM/TSP patients and HTLV-1 asymptomatic carriers with a high HTLV-1 proviral load equivalent to those of HAM/TSP patients [8], this finding suggested that HTLV-1-infected cells having the characteristic of Th1 increased in HAM/TSP patients. This evidence concerns the gene IFNG and tropical spastic paraparesis.