Dear All, This is a question about the effect of solvent fraction estimation on the quality of the initial phases. I am trying to solve the structure of a membrane protein at moderate resolution 3.5A from a MAD data set (peak, inflection and high energies). I have one copy of my complex which gives me a solvent fraction of 0.7 without considering the contribution of the detergent. With a related molecule I found a Phaser-MR solution but it does not refine although it is right. I am using Phenix with the phases obtained from molecular replacement and combining them with the MAD signal to locate the seleniums. A visual inspection of the anomalous fourier difference map shows 6 sites out of the 7 expected at 4sigma contouring. If I let Phenix decide by itself it goes on its own with two copies in the ASU and considers a solvent fraction of 0.50; then it finds 7 sites (and not 14 ? why is that?) (the seventh is weak but we expect this for this region of the complex). now if I force Phenix to take ncs_copies=1 and solvent fraction=0.7 then it only find 6 sites. Wether it is 2 "freely chosen" molecules/ASU and 0.5 of solvent or 1 "forced" molecule/ASU and 0.7 of solvent, the 6 first sites are identical (although occupancies are quite higher in the first case) Should I take into account the detergent and lower somewhat my solvent fraction (in the 0.6 range maybe) with the fixed one copy ASU estimation or just trust Solve/Resolve when it runs the density modification part. Does it really affect the quality of the phasing? Between these two extremes I am surprised to see that the quality of the resolve map are quite similar apparently showing clear new structural features. Thanks in advance for your comments, Pascal F. Egea, PhD Post Doctoral Researcher University of California San Francisco Department of Biophysics and Biochemistry Robert Stroud Laboratory [email protected]