Some PDB geometries have two or more possible residues at a given site (structural disorder). These are indicated by the letters 'A', 'B', etc., after the three letter abbreviation for the residue. Only one set of residues can be selected. To select all residues of type 'A,' use ALT_R=A; to select all those of type 'B,' use ALT_R=B, etc. Sometimes a space is used instead of 'A', in which case the keyword would be "ALT_R= " (note the space after the equals sign).