Proteins do almost everything in the body. They carry oxygen, fight infections, switch genes on and off, and pass signals between cells. The full set a human body makes – its proteome – is what most of modern medicine is ultimately trying to read.

The count of proteins scientists can measure in a sample of blood has been climbing for years: from hundreds, to thousands, to around ten thousand today, roughly half the body’s protein-coding genes. Put like that, the job sounds nearly done. Keep improving the technology, add proteins, and eventually the list will be complete.

It doesn't work that way. The proteins we still cannot measure are not a random leftover waiting to be picked up on the next pass. They are a specific kind of protein, absent for particular reasons. And several of those reasons point straight at the biology we most want to understand.