While there is a lack of correlation between CTL frequency and HTLV-1 PVL in HAM/TSP patients, there is a negative correlation between HTLV-1 PVL and CTL mediated cytotoxic potency by which the expression of CTL cytolytic components such as perforin and granzyme is accompanied with lower PVL. Here, PRF1 is linked to tropical spastic paraparesis.