All records in a PDB coordinate entry must appear in a defined order. Mandatory record types are present in all entries. When mandatory data are not provided, the record name must appear in the entry with a NULL indicator.
Optional items become mandatory when certain conditions exist.
JRNL Optional Mandatory if a publication describes the experiment. HET Optional Mandatory if non-standard group other than water appears in the entry. Optional Mandatory if the complete asymmetric unit must be generated from the given coordinates using non-crystallographic symmetry. ATOM Optional Mandatory if standard residues exist. PDB will follow standard procedures whenever format changes are proposed.