The mRNA codon that usually signals the beginning of protein synthesis is AUG.

This codon codes for the amino acid methionine, and it's often referred to as the "start codon".

Here's why it's important:

* Initiation of Translation: The ribosome, which is responsible for reading the mRNA and building the protein, recognizes AUG as the signal to start protein synthesis.

* Methionine as the First Amino Acid: In most cases, the first amino acid in a protein is methionine, encoded by AUG. This methionine might be removed later in the process.

However, there are exceptions:

* Alternative Start Codons: In some cases, other codons like GUG and UUG can act as start codons, although AUG is the most common.

Let me know if you have more questions!