Recently, the availability of the first reported monoclonal antibody (mAb), 4D4-F3, isolated in our laboratory, allowed us to study the expression, localization, and interaction in vivo of endogenous HBZ in HTLV-1-infected ACs, ATL and HAM/TSP patients (Raval et al., 2015; Baratella et al., 2017b). The gene discussed is HBZ; the disease is tropical spastic paraparesis.