Rex proteins are phosphorylated and are localized in the nucleus of infected cells. While some studies show that there might be some proteins encoded in this region, much of the region can be removed with no deleterious effects on replication. Homology between the sequences is highest in the tax and rex genes, and lowest in the LTR and nontranslated region. HTLV envelope proteins appear unable to sustain many mutations without becoming non-functional. Parenteral transmission takes place via contaminated blood or tissue such as through transplants or shared intravenous drug needles.
Sexual transmission places the receptive partner at the greatest risk.
Having multiple sexual partners is considered a very high risk behavior. The cellular receptor for HTLV is unknown. It is likely that HTLV can penetrate and infect many different cell types, however productive infection is observed in only a few cell types. HTLV establishes a lifelong persistent infection which usually remains subclinical, but occasionally produces disease after an incubation period ranging from ten to forty years. Clinical manifestations vary.