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Fans of science fiction will find a few items in this issue sure to trip the imagination. First, Tina Hesman Saey describes a discovery akin to something out of Alien: roving cancer cells that move from victim to victim, sneaking into others’ bodies to produce more of themselves. Saey, a lover of science fiction herself, calls it “cancer as parasite” or — as one researcher put it — extreme out-of-body metastasis.