`NOZERO ZERO`

In a PDB file, when the first residue in a protein chain has a negative number there is the possibility that a residue has the number zero. In some PDB files, the residue following "-1" is "1", not "0". If this situation occurs, and the original residue sequence has to be regenerated, special action must be taken to ensure that the residue following "-1" is correctly identified. If it should be "0", then keyword ZERO must be present, if it should be "1" then keyword NOZERO must be present.

These keywords should not normally be used. They are only needed when the special conditions just described are met. If these conditions are met, a message explaining that one of these keywords must be selected and used in the job.