Unknown amino acids "UNK".

May 2024, RCSB has 1,638 hits for UNK:
  1,109 Protein only
      0 Nucleic only
     420 Protein/NA
     105 Protein/Oligosacc
  ------
  1,634 √

Above results are consistent with UNK being protein only, unknown nucleotides being N.

See examples in Notes under Counting Amino Acids.