In addition, HTLV-1 Tax was shown to activate the t-bet gene with reduced expression of FoxP3 in the infected cells, resulting in their differentiation toward Th1 in HAM/TSP (Yamano et al., 2009; Yamamoto-Taguchi et al., 2013; Araya et al., 2014). This evidence concerns the gene TBX21 and tropical spastic paraparesis.