Four-wavelength MAD electron density (a) before and (b) after removal of Fourier ripples at the tungsten-cluster sites as discussed in the text. The phases were calculated using SHARP version 1.4 (a) and 2.0 (b), respectively, and subsequently optimized by density modification within SHARP. The model was not included in the phase calculations and the data was not sharpened. The density is contoured at 1σ. The loop 431–447 binding cluster 3 is shown (C atoms in yellow, O atoms in red, N atoms in blue, W atoms in white). The electron density is shown within 2.5 Å of the atoms.