One astonishing feature of cancer is that any individual specimen of cancer has a permutation of mutations that is unique to it. One woman’s breast cancer can have mutations in, say, thirty-two genes; the second woman’s breast cancer can have sixty-three, with only twelve overlapping between the two. The histological, or cellular, appearance of two “breast cancers” may look identical under the pathologist’s microscope. But the two cancers may be genetically different—they behave differently and may require radically different therapies.
