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(THE CONVERSATION) I had assumed that the small lump in my breast was a blocked milk duct from nursing my seven-month-old son. The news that I had stage 2 breast cancer stunned.

“But it’s not in my family,” I told the radiologist. “And I have a healthy lifestyle! Why did I get breast cancer?”

In one way or another, friends and relatives here in the U.S. asked the same question. Why had this happened to me? Their explanations coalesced around a single point: bad genes.

But when I to...