T-cells have also been reported to be affected in HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP) IFN-treated patients; the frequency of perforin expression in CD8+ T-cells was significantly decreased after treatment in HTLV-infected patients who experienced clinical improvement, while it was increased in patients who did not experience such an improvement [117]. Here, PRF1 is linked to tropical spastic paraparesis.