What is the difference between the entity_src_genCategory tag and the entity_src_natCategory tag in pdb-xmls?
src_nat = isolated directly from a natural tissue

src_gen = the source was genetically manipulated

The files that you use (PDBML/XML) are derived from the primary PDB data format which is PDBx/mmCIF.
The names in XML seem to correspond to mmCIF names, so you can read documentation on http://mmcif.wwpdb.org/, and here are direct links:


category entity_src_nat
category entity_src_gen


Can they occur together in one file? I suppose they can for complexes: one "entity" could be natural and another genetically manipulated.