How is polymorphism/microheterogeneity described?
A. It is well recognized that this is not well represented by PDB format. The current practice is described here. Although microheterogeneity does not present a problem within the coordinate records, it does introduce a difficulty in the specification of the sequence in the SEQRES records where only a single residue may be specified for each sequence position. In cases where a single sequence position is modeled as different residues and these residues differ with respect to occupancy, then the residue with the higher occupancy is used to the define the SEQRES sequence. If the different residue models cannot be distinguished by occupancy, then the SEQRES sequence is defined using the residue which matches the sequence obtained from the sequence database reference. For example, residue 60 has two isoforms (SER and VAL) modeled with equal occupancies; however, residue SER matches the sequence database reference. Residue SER is listed in the SEQRES, since it matches the sequence database
