Integral polytopic membrane proteins contain only two types of folds in their transmembrane domains: xcexb1-helix bundles and xcexb2-barrels. The increasing number of available crystal structures of these proteins permits an initial estimation of how sequence variability affects the structure conservation in their transmembrane domains. We, thus, aim to determine the pairwise sequence identity necessary to maintain the transmembrane molecular architectures compatible with the hydrophobic nature of the lipid bilayer.