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Within the testicles of every man, a hitherto unknown process is going on that vastly increases the odds of any children they father having certain kinds of mutations. The risk increases rapidly as men age. A kind of benign testicular cancer turns sperm stem cells selfish: They start churning out far more copies of themselves than they would do normally, and the mutant sperm they generate begin to take over. As men age, their family jewels are slowly and stealthily becoming tarnished.