The sperm. Yes, really.

So the £19 glass-skin serum on your feed is built on salmon sperm, or milt, if you'd rather say that at dinner. Manufacturers use it because the DNA fragments are unusually clean and close enough to ours that skin cells accept them without complaint. Trout gets used too, and VT's version comes from Korean ginseng if you'd rather not think about it at all.

Whether any of that DNA gets through your skin from a serum is the better question, and we answered it in August. Short version: look for Sodium DNA high on the ingredient list, and don't overpay. Most PDRN serums are inexpensive, and a high price should be justified by something else in the formula.