Definition of the transition mechanism from the native globular protein into fibrillar polymer was greatly improved by the biochemical and biophysical studies carried out on the two amyloidogenic variants of human lysozyme, I56T and D67H. Here we report thermodynamic and kinetic data on folding as well as structural features of a naturally occurring variant of human lysozyme, T70N, which is present in the British population at an allele frequency of 5% and, according to clinical and histopathological data, is not amyloidogenic. This variant is less stable than the wild-type protein by 3.7 kcal/mol, but more stable than the pathological, amyloidogenic variants. Unfolding kinetics in guanidine are six times faster than in the wild-type, but three and twenty times slower than in the amyloidogenic variants. Enzyme catalytic parameters, such as maximal velocity and affinity, are reduced in comparison to the wild-type. The solution structure, determined by 1 H NMR and modeling calculations, exhibits a more compact arrangement at the interface between the -sheet domain and the subsequent loop on one side and part of the ␣ domain on the other side, compared with the wild-type protein. This is the opposite of the conformational variation shown by the amyloidogenic variant D67H, but it accounts for the reduced stability and catalytic performance of T70N.Amyloidosis is an emerging category of diseases characterized by the extracellular accumulation of protein aggregates that share a common fibrillar conformation. The 20 proteins that can generate amyloid deposits in humans are extremely heterogeneous in function and structure, but, along the pathological transformation leading to aggregation and precipitation, all of them exhibit the same peculiar conformational pattern named cross- structure, irrespective of the parentstarting arrangement (1). The lack of any sequence similarity and folding analogy among the amyloid-forming proteins led Dobson to conclude that the ability to form cross- structure, wherein hydrogen bonds are formed between polypeptide chains in directions parallel to the fiber axis, is a generic property of polypeptide chains (2). Investigations of structure (3-4), folding dynamics (5-7), and fibrillogenesis (3, 8) of the initially reported amyloidogenic variants of lysozyme have made important contributions to a better understanding of the process involved in the conversion of globular proteins into amyloid fibrils. Amyloidogenic lysozyme represents probably the most convenient and informative model of fibrillogenesis from a globular protein. Besides being, in fact, one of the best characterized enzymes, its fibrillogenic mechanism is not influenced by protein fragmentation; nor, to our knowledge, does the wild-type species generate amyloid deposits in vivo, even in the elderly. Thorough analysis of several biochemical properties of the amyloidogenic variants in comparison to the wildtype species showed that pathogenic lysozymes are less stable than wild-type (3-8). This thermodynamic destabilization c...