The first a-helix of the BLV LHR ends at Asp406, and between the end of this helix and the beginning of the next is a threeresidue linker, comprised of Leu407, Gln408 and Pro409. Although Leu407 is positioned such that it faces the groove between N-helices, a ridge across this groove prevents Leu407 from docking particularly deeply (Fig. 1C ). This Leucine is not conserved among leukaemia viruses. Instead, there is an Arginine residue at this position of the HTLV-1 LHR that makes contact with the coiled coil. Such differences may account, in part, for the specificity and significant differences in potency that are observed for the BLV and HTLV-1 peptide inhibitors [17] . The conserved proline (BLV Pro409) of the extended non-helical linker induces a sharp kink in the LHR thereby allowing a second a-helix to bind almost directly across the groove, at right angles to the rest of the LHR. Crucially, this second helix is not resolved in the published HTLV-1 TM structure [20] . At the point at which this change of direction occurs, a conserved leucine, Leu410, docks into the start of a deep hydrophobic pocket. Adjacent to Leu410 in this pocket and within the second LHR helix is Val414, which is also conserved in HTLV-1 (Fig. 1C, D) . Substitution of either Leu410 or Val414 with alanine markedly impairs the fusogenic function of envelope (Fig. 2C) . The substitution of L410A is somewhat more detrimental than substitution of Val414, with an activity loss of 75% and 65% respectively.