Hi i am refining a protein strucutre with space group P61. Intesity statistics doesnot suggest any twining but refmac and phenix xtriage suggest a twin operator with fraction 0.48. when refined with this twin operator R factors come dowm by 3-5% and also map looks much better. but upon closer inspceion it seems that maps are heavily biased toward the model in both refmac and phenix ( I have mutated one lysine to his and it changed the map to cover his). my question is why twin refinement maps are biased and what are the ways to overcome this. my second question is how to decide whether to use twin refinement or not. is it depends on R factor statistics or we have to check some other parameters. thanks in advance subhash bihani scientific officer Bhabha Atomic Research Centre Trombay-400085 Mumbai(M.S.) India Ph. +912594688 (o) +919322191020 -- Subhash Bihani Scientific Officer Solid State Physics Division Bhabha Atomic Research Centre Trombay, Mumbai-85 Ph. 25594688 (o) alternate email: [email protected]