SAD phasing: History, current impact and future opportunities.

Single wavelength anomalous diffraction (SAD) can trace its beginnings to the early 1950s. Researchers at the time recognized that SAD offers some unique features that might be advantageous for crystallographic phasing, despite the fact that at that time recording accurate SAD data was problematic. In this review we will follow the trail from those early days, highlighting key advances in the field and interpreting them in terms on how they stimulated continued phasing development that produced the theoretical foundation for the routine macromolecular structure determination by SAD today. The technological advances over the past three decades in both hardware and software, which played a significant role in making SAD phasing a 'first choice method', will also be described.