Identical twins Kelly McCarthy and Kristen Maurer share a lot more than a maiden name. Only a few months after McCarthy received her breast cancer diagnosis in December of 2011, Maurer received one as well. But McCarthy’s case was more aggressive than her sister’s, and it demanded more advanced treatment. So when it came time for doctors to reconstruct McCarthy’s breasts, her sister volunteered the skin and fat tissue from her own body.

"It wasn't a question, she didn't have to ask me," Maurer, a college enrollment counselor, told the Associated Press. Along with her sister, a nurse, the 34-year-old pair lives in Crown Heights, Ind. "Having a twin is very like having a child. You would do anything for them ... in a heartbeat."

Identical twins begin as a single egg that splits in two. Short of having matching fingerprints, this means they are perfect genetic matches, as they share the same DNA. Over the years, science has made great leaps in understanding what effects this can cause. Identical twins are more likely to have the same reading disabilities, the same risk for autism, Alzheimer’s disease, schizophrenia, and to a lesser extent, alcoholism and bipolar disorder.

And like McCarthy and Maurer, identical twins share an increased risk for breast cancer — about three to four times the risk of the average woman, according to estimates from Cancer Research U.K. While one out of every eight women is generally at risk for breast cancer, one out of every three women who is an identical twin should expect to develop the disease. McCarthy and Maurer report that breast cancer doesn’t run in their family, although their mother died of colon cancer last year.

Treatment for McCarthy’s form of breast cancer — triple-negative cancer, which means it isn’t fueled by any of the three normally checked hormones — required multiple approaches. She was nine months pregnant at the time. A week after giving birth, she underwent chemotherapy, radiation treatment, and needed full removal of the fatty tissue in both breasts, otherwise known as a double mastectomy.

Maurer would later undergo her own double mastectomy because her sister’s diagnosis was so severe; however, she did not have to go through chemotherapy or radiation. Still, she said her sister took the follow-up diagnosis hard.

"Kelly was more upset than I was during my diagnosis,” she said. “And likewise, when she was diagnosed I was a mess.”

But when the opportunity arose for Maurer to donate a portion of her skin and fatty tissue for her sister’s breast reconstruction surgery, because McCarthy didn’t have enough herself, the decision was a no-brainer. The surgery normally relies on fat taken from a patient’s abdomen, buttocks, or thighs. Maurer underwent a sort of “tummy tuck” for the donation, as the pair’s doctor, Dr. David Song, chief of plastic and reconstructive surgery at the University of Chicago Medical Center, removed fat from her abdomen to rebuild McCarthy’s chest.

Typically, Song said, identical twins will experience a “mirroring effect,” which makes it very unlikely only one breast will be affected.

Tuesday’s surgery proceeded without a hitch, the doctor reported. Both sisters are making full recoveries. After a lifetime of sharing memories growing up, the sisters now say the cancer has brought them closer together — in more ways than one.

"I feel closer,” McCarthy told the AP. “Her tissue is over my heart."