To provide direct evidence that HIV-1 infection of epithelial cells was mediated by HTLV-1 Env, we next performed neutralization assays with antibodies against HTLV-1 Env (anti-HTLV-1 Env gp46 monoclonal antibodies [MAbs] PRH-7A and PRH-4 and IgG from patients with HTLV-1-associated myelopathy/tropical spastic paraparesis [HAM serum IgG]). This evidence concerns the gene SERPINH1 and tropical spastic paraparesis.