Dear Editor,A multi-domain protein consists of two or more well-folded domains connected by flexible linkers, which may lead to large scale inter-domain motions related to the protein function.Therefore, it is appropriate to represent a flexible multi-domain protein by an ensemble of structures containing multiple conformational states (Beadó and Blackledge,2010).Although X-ray crystallography is currently the most popular technique for structure determination, it would be rather challenging to solve structure of such a flexible multidomain protein since it is hard to obtain high-quality crystals if the protein does not stabilize in a dominant conformation.Solution nuclear magnetic resonance (NMR) is able to investigate structure and dynamics of flexible multi-domain proteins with modest sizes, but it may not be easy to obtain enough long-range NMR restraints in order to determine orientations between the domains.Small-angle X-ray scattering (SAXS) has made substantial progress over the past decades (Graewert and Svergun, 2013), which can provide valuable structural information, such as the sizes and shapes, of proteins.SAXS is particularly useful in characterizing the flexibility of a multi-domain protein because the scattering profile contains the information of multiple conformations of the protein and their relative population in solution.Although the resolution of SAXS is inherently low since the complex protein structure is reduced to a onedimensional orientationally averaged profile, it can serve as a complementary experiment to these high-resolution techniques.For example, structures of individual domains can be solved by NMR, and SAXS provides a restraint between the domains (Grishaev et al., 2005).