The different views from small angles.

T he small-angle scattering of xrays or neutrons from proteins in solution can provide important information about the structure of the protein and the nature of interactions or distance correlations among the protein molecules (1, 2). The former is encoded in the form factor [P(q)], and the latter in the structure factor [S(q)]. These functions are of great interest to the structural biology community; the form factor can be used to develop and test three-dimensional structural models of proteins (3, 4), whereas the structure factor can inform efforts to crystallize proteins for highresolution structural analysis by providing insights into their organization in solution (4, 5). Because the measured small-angle scattering profile [I(q)] from a solution of particles is proportional to the product of the ensemble and rotationally averaged form factor and the structure factor [P(q)S(q)], accurate extraction of the two contributing functions, and their subsequent interpretation, is complex. The work of Shukla et al. reported in this issue of PNAS (6) is aimed at resolving controversy with regard to the interpretation of small-angle scattering data from solutions of the well studied protein lysozyme. The specific issue examined concerns the interpretation of the extracted S(q) in terms of intermolecular interactions among lysozyme molecules. The conclusion drawn by Shukla et al. contradicts previously published interpretations of similar data and is boldly presented as their title: ‘‘Absence of equilibrium cluster phase in concentrated lysozyme solutions.’’