Given that in vitro culture of HAM-PBMCs is known to induce the expression of HTLV-1–encoded genes, particularly HTLV-1 Tax (13), we examined the relationship between the expression levels of the RGMA gene and those of HTLV-1−encoded genes, Tax and HBZ, under cell culture conditions using qRT-PCR. The gene discussed is HBZ; the disease is tropical spastic paraparesis.