Classically NK cells are divided in CD56dim or CD56high based on their functions: CD56dimCD16high NK cell subset expresses high levels of perforin and mediates natural and antibody-dependent cellular cytotoxicity, and enhanced killing while CD56highCD16± NK cells are characterized by low levels of perforin, and are primarily specialized for cytokine production, a function that could be deregulated as a consequence of inflammatory status of HAM/TSP disease. Here, PRF1 is linked to tropical spastic paraparesis.