Think of a peptide like a necklace. 🧵 The beads threaded on the main string are called residues. Retatrutide’s main chain contains 39 beads, so it is counted as a 39-residue peptide. ✨ What makes this chain special? • Positions 2 and 20 are Aib, a nonstandard amino acid. • Position 13 is MeL, another modified building block. • Position 17 is lysine (Lys), which carries a special side-chain attachment. 🔍 Here’s the part that causes confusion: Attached to Lys17 is a branch made of an AEEA spacer, a γGlu linker, and a C20 diacid lipid tail. The γGlu may look like an “extra amino,” but it hangs off the side of bead 17. It is not threaded into the main backbone. Picture a charm hanging from a necklace: the charm changes the necklace, but it does not add another bead to the string. ✅ Bottom line: 39 backbone residues + a side-chain modification at Lys17 = still 39 residues, not 40. Retatrutide is called a triple agonist because it is designed to engage the GLP-1, GIP, and glucagon receptor pathways. These illustrations show just how much careful engineering can fit into one small peptide. Quick quiz: Does the γGlu make retatrutide a 40-residue peptide? Drop your answer below! 👇 For education only. This post does not provide dosing or medical advice.