“Hair isn’t made of cells,” says Leona Samson, professor of biology and biological engineering and an affiliate in the Koch Institute for Integrative Cancer Research. “The stuff that makes hair”—a thready protein called keratin that also forms fingernails—“is produced by cells in the hair follicle. You have to have cells to have cancer.” Though rare, tumors do sometimes form in the cells in the hair follicle. These cancer cells have lost their ability to grow in a way that is appropriate for where they are in the body. A liver cell, for example, may become so deranged that it doesn’t act like a liver cell anymore. Then, when it divides (as cells do), it passes its derangement on, replicating its abnormalities and spreading them.