Single-wavelength anomalous diffraction (SAD) is a technique used in X-ray crystallography that facilitates the determination of the structure of proteins or other biological macromolecules by allowing the solution of the phase problem.
Compared to MAD, SAD has weaker phasing power and requires density modification to resolve phase ambiguity.
[1] This downside is not as important as SAD's main advantage: the minimization of time spent in the beam by the crystal, thus reducing potential radiation damage to the molecule while collecting data.
SAD also allows a wider choice of heavy atoms and can be conducted without a synchrotron beamline.
[1] Today, selenium-SAD is commonly used for experimental phasing due to the development of methods for selenomethionine incorporation into recombinant proteins.