The results of the present study may indicate that the p75NTR Ser205Leu polymorphism promotes an improvement in the functions of the p75NTR receptor and optimizes Trk-NGF-p75NTR binding; this would lead to better NGF activity, which would contribute to avoiding the development of HAM in some infected patients. The gene discussed is NTRK1; the disease is tropical spastic paraparesis.