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(THE CONVERSATION) Anyone who came of age in the 1990s remembers the “Friends” episode where Phoebe and Rachel venture out to get tattoos. Spoiler alert: Rachel gets a tattoo and Phoebe ends up with a black ink dot because she couldn’t take the pain. This sitcom storyline is funny, but it also simply illustrates the question that I and many others in the field of “paingenetics” are tryingtoanswer. What is it about Rachel that makes her different from Phoebe? And, more importantly,...