This article seems to contain a few errors. Codons are found on the messenger RNA. Therefore, they can’t contain thymine. They must have uracil, instead. The RNA codon AUG (your ATG) is the only codon for methionine. If it is the “start” codon, how is methionine coded?

Nicholas L. Reuter
University of Cincinnati
Cincinnati, Ohio

Most geneticists and genetics textbooks refer to both DNA and RNA triplets as codons. The AUG mRNA codon codes for methionine and also acts as the start codon for all proteins .–The editors