At 71, She’s Never Felt Pain or Anxiety. Now Scientists Know Why.

Ms. Cameron said she had been shocked by the interest in her case. Until her conversation with Dr. Srivastava, pain was not something she thought about. Perhaps it helped that even though she burned and cut herself quite often, her injuries rarely left scars — something else that scientists believe is connected to the mutation.

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A number of articles have been written about parents of children with similar conditions. Many live in fear that without pain, their children won’t learn how to avoid hurting themselves. Her parents never made it an issue, she said. She suspects this may be because she inherited the mutation from her father.

“I can’t remember him needing any painkillers,” she said. “I think that’s why I didn’t find it odd.”

Unfortunately, because he died before the discovery, it will remain unknown whether he carried the mutation. Her mother does not share it. Neither does her daughter. Her son “has the same microdeletion in FAAH-OUT, but does not have the other mutation that confers reduced FAAH function,” Dr. Cox said.

In other words, her son shares some, but not all, of her pain insensitivity.

Scientists are also intrigued by Ms. Cameron’s extraordinarily low anxiety level. On an anxiety disorder questionnaire, she scored zero out of 21. She cannot recall ever having felt depressed or scared.

“I am very happy,” she said.

In retrospect, she sees how her genetic disposition may have aided her at work. After years as a primary-school teacher, she retrained to work with people with severe mental disabilities. Erratic, aggressive behavior never riled her, she said.