Skin that cannot tolerate light

In erythropoietic protoporphyria a faulty enzyme lets light-reactive porphyrins build up in the skin, and sunlight turns them into a burning pain within minutes. The implant raises eumelanin so less light reaches those molecules, and people gain usable time outdoors instead of a life arranged around shade. This is the use the molecule was built into a medicine for, and it is handled by specialist centres. It is the clearest demonstration that the pigment it makes does real physical work.