To use contrast variation, different components of a system must scatter differently. This can be based on inherent scattering differences, e.g. DNA vs. protein, or arise from differentially labeled components, e.g. having one protein in a complex deuterated while the rest are protonated. In terms of modelling, small-angle X-ray and neutron scattering data can be combined with the program MONSA. An example in which SAXS, SANS and EM data has been used to build an atomic model of a large multi-subunit enzyme has recently been published. For some examples of this method see.