Hi Honglue,
By default, programs in 3DNA (DSSR and SNAP as well) take the alternative atomic positions
'A' or
'1' if present (i.e. other than space
' ') into consideration. See the configure file
$X3DNA/config/misc_3dna.par:
# Section 2: alternative location in ATOM or HETATM records
# default to A or 1 (A1), and ' ' is always added
<alt_list>A1</alt_list>
You can change the default by directly editing the
misc_3dna.par file for global effects, or more conveniently by specifying the
-alt_list option from the command line on a case-by-case basis. E.g.,
-alt_list=B if you want to analyze the other alternative positions (
'B' as in your sample PDB file). See the documentation on PDB "Coordinate File Description" at
http://deposit.rcsb.org/adit/docs/pdb_atom_format.html on the alternate location indicator.
HTH,
Xiang-Jun