The mechanism of insertion of the gp41 fusion domain into the T-cell’s lipid bilayer is clearly of paramount importance in understanding the process of membrane fusion but the structure of the fusion domain has been controversial with several studies directly contradicting each other. NMR studies of the gp41 fusion domain in solution in lipid micelles revealed an α helical structure but NMR studies of the fusion domain in solid state in lipid bilayers indicate a structure which is primarily composed of β sheets. (NMR is a widely used spectroscopic technique which utilises the oscillations of nuclei in a strong magnetic field. This technique is covered in the TSMB course but current PPS students will not be able to follow the link.) Neither of the sets of conditions is close enough to physiological conditions to be conclusive and so a recent study, (Lai, A.L. et al., (2012)), was designed to examine the structure taken by the domain under a range of possible physiological conditions.