A four amino acid (Glycine – Glutamic acid – Lysine – Glycine) peptide that belongs to the group of signal peptides that mimic skin-protein breakdown products to trick the skin into creating some nice new skin proteins, such as collagen (if this sentence is not clear, here is a bit longer and nicer explanation). 

According to its manufacturer, Tetrapeptide-21 shows superior collagen, hyaluronic acid and fibronectin (all nice and important skin elements) boosting activity, and can smooth and minimize all kinds of wrinkles. Their in-vivo (made on real people) tests confirmed that the peptide improves skin elasticity after 8 weeks of use, and they also have some nice before and after pictures showing visible-to-the-naked-eye wrinkle reduction around the eye area.