Unknown amino acids "UNK".

UNK is "unknown amino acid":
https://www.wwpdb.org/documentation/file-format-content/format33/sect4.html#HET
N is unknown nucleic acid. UNL is unknown ligand.

UNK is always ATOM, or at least I haven't found HETATM UNK.
Therfore UNK is a standard amino acid.
UNK is typically modeled as Ala or main chain only or alpha carbons only.
Regardless, they are (as of FG 4.4) labeled S-, although that is inaccurate
for any UNK that are actually Gly or Ala (when modeled as Ala).

When UNK has no alpha carbon, FG MIT reports it only as amino acids with no alpha carbons.
Example: 1b8d.

March 2025: 1,767 so evidently new entries are using 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.