SAD or MAD phasing: location of the anomalous scatterers

The method of joint probability distribution functions is applied in order to estimate the structure-factor moduli of the anomalous scatterer substructure both in the SAD (single-wavelength anomalous dispersion) and in the MAD (multi-wavelength anomalous dispersion) cases. The experimental data \F(1)(+)\,\F(1)(-)\,...,\F(n)(+)\,\F(n)(-)\ measured at n wavelengths are used simultaneously to estimate the value of \F(oa)\ arising from the normal scattering of the anomalous scatterers. A practical procedure is described that, when applied to the experimental diffraction data of several proteins, shows robustness and efficiency.