Lixia Jin wrote: > I have two chains in my model: A chain has 324 residues. I want to > modify only part of chain A (193-249) and chain B. > > How do I select the residues?

See http://salilab.org/modeller/9v1/manual/node204.html

In your case, to select all of chain B, and residues 193-249 in chain A, from a model 'mdl', something like the following should work:

s = selection(mdl.chains['B'], mdl.residue_range('193:A', '249:A'))

Ben Webb, Modeller Caretaker