Hi Hongwei, You do not need to use symmetric copies of a molecule when fitting into a symmetric map. Fitting a single asymmetric unit works. I demonstrated fitting one copy of a molecule into a myosin thick filament map while two neighboring symmetrically placed copies move automatically to maintain symmetry. This has no effect on the fitting. It merely lets you see when a fit position causes a steric clash with a symmetrically place copy. The placement for the symmetric copies is described with matrices in the header of the PDB file. Here is a description of this if you want to try it: http://www.cgl.ucsf.edu/chimera/tutorials/volumetour/volumetour.html#symcopi... Tom