## **Core Concept**
The initiator codon, also known as the start codon, is a specific sequence on a messenger RNA (mRNA) that signals the initiation of protein synthesis. The most well-known initiator codon is **AUG**, which codes for the amino acid **methionine** in eukaryotes and **formylmethionine** in prokaryotes.

## **Why the Correct Answer is Right**
The correct answer is **Methionine** because the initiator codon **AUG** specifically codes for methionine in eukaryotes. This codon is universally recognized as the start signal for protein synthesis in all organisms. When the ribosome encounters AUG during translation, it begins to assemble the amino acids into a polypeptide chain, starting with methionine.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Histidine* - This amino acid is coded by the codons CAU and CAC, not by the initiator codon AUG.
- **Option B:** *Glutamine* - Glutamine is coded by the codons CAA and CAG, not by the initiator codon AUG.
- **Option D:** *Valine* - Valine is coded by the codons GUU, GUC, GUA, and GUG, not by the initiator codon AUG.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while **AUG** codes for methionine in eukaryotes, in prokaryotes, it codes for **formylmethionine**. This difference is crucial for understanding the initiation of protein synthesis in different types of organisms.

## **Correct Answer:** . Methionine