Two key residues contained in the HTLV SU RBD and conserved between HTLV-1 and -2, arginine 94 (Arg94) and serine 101 (Ser101) for HTLV-1 Env which correspond to Arg90and Ser97 in HTLV-2 Env, have been shown to alter cell-to-cell fusion and infection when mutated [36,37]. The gene discussed is ERVW-1; the disease is infection.