Step 1: List the genetic code's stop signals.
Translation terminates when the ribosome encounters one of exactly three codons: UAA, UAG, or UGA, none of which code for an amino acid.
Step 2: Check the remaining codon.
AUG does not appear in that stop-codon set. Instead, AUG codes for methionine and additionally serves as the near-universal start codon that initiates translation.
Step 3: Draw the conclusion.
Since AUG signals the beginning of protein synthesis rather than its end, it is the one codon among the four that is not a stop codon.
\[ \boxed{\text{AUG}} \]