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Alternate locations
From Proteopedia
Some PDB files have alternate locations (altlocs) for some atoms. Each atom that has altlocs is marked with an altloc identifier, typically A and B.
The OCA Database and Browser highlights the structure with the most 'altlocs', see Believe It or Not! within Proteopedia for information mined from OCA.
Note
It looks like in the mmCIF format correspondence may be 'alt_id'. However, because that is listed as corresponding to 'alt_loc' , it is unclear if that is the same as 'altLoc' listed in the PDB format guide.
