It's common to have multiple ATG codons in an mRNA sequence. Generally, the first ATG serves as protein translation starting site and is considered as a start codon if that ATG is at the beginning of a full and functional open reading frame.

How many start codons are there?

The findings, to be published on February 21, 2017, in the journal Nucleic Acids Research by scientists in a research collaboration between NIST and Stanford University, demonstrate that there are at least 47 possible start codons, each of which can instruct a cell to begin protein synthesis.

Are there 3 start codons?

The list of amino acid abbreviations is located below the table. AUG, as the start codon, is in green and codes for methionine. The three stop codons are UAA, UAG, and UGA. Stop codons encode a release factor, rather than an amino acid, that causes translation to cease.