HBZ and tropical spastic paraparesis: Although further studies are needed to clarify the reason for this difference, it is possible that both the viral subgroup and host genetic background (including HLA type) determine the anti-HBZ antibody response, since our recent study indicated that different HTLV-1 subgroups are characterized by different patterns of viral and host gene expression in HAM/TSP patients via independent mechanisms of direct transcriptional regulation [36].