SS-31 is four amino acids long. That's it. Four. And the exact same molecule sits inside a medicine the FDA approved in 2025. So what is SS-31 when it's in a research vial? Most people figure it's that medicine, just without the pharmacy. It isn't.
What is SS-31? A molecule, not a medicine
SS-31 is a short, lab-built peptide designed to reach the inside of mitochondria, the tiny power plants in cells. Its official drug name is elamipretide. But an FDA approval doesn't belong to a molecule. It belongs to one finished product with one label.
The proof: read what the FDA actually approved
Let's get into it.
The name is just two scientists. SS stands for Szeto-Schiller, after Hazel Szeto at Weill Cornell in New York and Peter Schiller at the Montreal Clinical Research Institute. SS-31 is one of a family of small peptides built to reach the inner wall of mitochondria. Szeto's 2014 review, which sums up years of animal studies, describes it grabbing onto cardiolipin, a fat molecule that helps hold the folds of that wall in shape (Szeto, Br J Pharmacol 2014). Picture the staples that keep a pleated curtain pleated.
It's built from unusual parts. PubChem writes it as D-Arg-Dmt-Lys-Phe-NH2 (PubChem). The arginine is the D form, the mirror image of the one ordinary proteins use. Dmt is dimethyltyrosine, a tyrosine with two extra methyl groups bolted on. The whole thing weighs 639.8.
Then one drug product got approved. On September 19, 2025, the FDA granted accelerated approval to one elamipretide medicine for Barth syndrome, a rare inherited disease of the mitochondria, in patients weighing at least 30 kg (FDA, 2025). Accelerated means the yes rests on a stand-in measure, knee muscle strength here, and keeping it can depend on a confirming trial (Drugs@FDA).
Read the label and it's a product, not a powder. The approved medicine is a ready-made liquid with a preservative and other ingredients in it. Its ingredient is listed as elamipretide hydrochloride, the peptide paired with a salt, at 749.2. The approval covers that exact product. Not the molecule wherever it turns up.
So why should you care? Because "same molecule as an FDA-approved drug" tells you nothing about a vial. What a vial can show you is identity. SS-31 weighs about 640, those two extra methyl groups included, and the LC-MS identity result on a certificate of analysis, or COA, is where you check it.
"So is research SS-31 FDA approved?" No. Drugs@FDA lists a single elamipretide application, for one product. A research vial isn't it.
The big learning lesson
Keep reading: MOTS-c vs SS-31, compared side by side · Why mitochondria matter, and the peptides studied with them · How to read a peptide certificate of analysis
Frequently asked questions
What is SS-31?
SS-31 is a lab-made peptide, four amino acids long, that binds cardiolipin on the inner membrane of mitochondria. The SS stands for Szeto-Schiller, the two scientists behind it.
Is SS-31 the same as elamipretide?
Same molecule, yes. Elamipretide is its official drug name, and one elamipretide medicine got FDA accelerated approval in 2025 for Barth syndrome. Research-grade SS-31 isn't FDA approved.
What is the molecular weight of SS-31?
PubChem lists SS-31 at 639.8. The approved medicine's label lists its hydrochloride salt form at 749.2, because the salt adds weight.