For us to live, parts of us must die. Every day, billions of our cells shrink, break up into small parcels, and get tidied away by other janitorial cells. This gentle, organised cellular suicide is called apoptosis, and we depend upon it. Our hands start off as solid lumps; it’s apoptosis that sculpts our fingers by killing off the cells in between them. Now and then, our cells threaten to grow out of control; it’s apoptosis that stops them from becoming tumours.

There are many ways of triggering apoptosis, and one route involves two large groups of proteins: the tumour necrosis factors (TNFs), and the receptors that they stick to. When they meet, they set off a chain reaction inside