[Chimera-users] Four letter residue names

Hi all A pdb file deriving from a mmCIF file (with chimerax) and comprising both nucleotides and proteins, shows the protein residue names with four characters (i.e., using also column 21), such as ALAM in one subunit, or GLNm in another subunit. I read on passing somewhere (and I am unable to trace that back) that chimera accepts such four character names but was unable to find a description from chimera help. I also got lost in deciphering the intricacies of mmCIF files. What is the scope of the column 21 character? Could that simply be ignored, or deleted, for use with the CHARMM FF? Thanks for advice francesco pietra