HTLV-1-infected CD4+ T-cells secrete IFN-γ (Hanon et al., 2000b; Araya et al., 2014), and the frequency of IFN-γ-secreting HTLV-1-specific CD4+ T-cells has been shown to be higher in patients with HAM/TSP than in asymptomatic HTLV-1 carriers with a similar proviral load (Goon et al., 2002). Here, IFNG is linked to tropical spastic paraparesis.