The start codon is AUG (adenine-uracil-guanine), which codes for methionine (Met) in eukaryotes and formylmethionine (fMet) in prokaryotes. It marks the beginning of the open reading frame (ORF). UAA, UAG, and UGA are the three stop codons (ochre, amber, and opal/umber respectively) — they signal termination of translation and do not code for any amino acid (in the standard genetic code).