The host genotype (determined mainly by the human leukocyte antigen (HLA) class I and killer cell immunoglobulin-like receptor (KIR) loci), quality of the cytotoxic T cell (CTL) response against HTLV-1-infected cells, T regulatory cell (Treg) frequency (forkhead box protein 3 (FOXP3)+) and PVL are important risk factors for the development of TSP/HAM [27–29]. The gene discussed is FOXP3; the disease is tropical spastic paraparesis.