The trimeric recombinant MBP-BLV-hairpin protein was purified by Superdex 75 gel filtration, and the required fractions pooled and concentrated to 12 mg ml 21 . Tris [2-carboxyethyl]phosphine hydrochloride (TCEP-HCl) was then added to a final concentration of 5 mM. Protein concentration was estimated by absorbance measured at 280 nm. Crystallization conditions were identified using the sitting drop vapour diffusion technique, with crystal screens from Hampton Research and Emerald Biostructures. Optimal crystallization was achieved by streak seeding with a reservoir solution composed of 24.5% (v/v) isopropanol, 13.5% (v/v) PEG-4000 and 0.1 M sodium citrate pH 5.6, and optimal diffraction was achieved using a cryoprotectant composed of mother liquor supplemented with 20% (v/v) glycerol. Diffraction data was collected at beamline BM14 at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The structure was solved by molecular replacement using a truncated model of MBP-HTLV Hairpin (PDB ID 1MG1). Refinement proceeded through cycles of model building using Coot and O [50, 51] and refinement using REFMAC5 [52] . Data and refinement statistics are given in Table 1 Peptides Peptides were synthesised using standard solid-phase Fmoc chemistry and unless stated otherwise have acetylated N-termini and amidated C-termini. The peptides were purified by reversephase high-pressure liquid chromatography and verified for purity by MALDI-TOF mass spectrometry. All peptides were dissolved in dimethyl sulfoxide (DMSO), the concentration of peptide stock solutions was confirmed by absorbance at 280 nm in 6M guanidine hydrochloride and peptides were used at the final concentrations indicated.