initiation codon /inish′ē·ā″shən kō″don/ [L, initium, beginning, caudex, book] , the triplet of nucleotides, usually adenine-uracil-guanine (AUG) or, in some cases, guanine-uracil-guanine (GUG), that in eukaryotes and archea code for methionine and in bacteria code for formylmethionine, the first amino acids in all protein sequences. Also called initiator codon, start codon.
