A  
Selenocysteine is inserted directly during translation via recoding of the UGA codon and is therefore not generated by chemical alteration of an existing amino-acid residue [Böck, 1991, PMID 1838215]. In contrast, triiodothyronine results from iodination of tyrosyl residues in thyroglobulin [Deme, 1978, PMID 655617], while hydroxyproline and hydroxylysine arise from post-translational hydroxylation of proline and lysine in collagen [Belostotsky, 2022, PMID 35055190].