When parents of five-year-old Charlotte Godish asked if she was willing to help her twin brother, Bradley, who has leukemia, she didn't hesitate.
Though she didn't fully understand the "medical process" she wanted to help.
Speaking to ABC News, dad Brian Godish said: "Her words were, 'Yeah, just let me know when you need me.'"
In January, Charlotte's parents approached her to donate her cells to Bradley.
Bradley had complained of sore feet around Halloween last year.
The twin's mum, Jennifer Godish, told People, "I'd just had the baby and Bradley kept saying that he wanted to be carried."
"I thought he was just a little jealous and wanted some extra attention."
Taking Bradley to the doctor, they were told he had a virus.
A few days later, while reading a Jodi Picoult novel, Jennifer read a paragraph mentioning a link between growing pains and leukemia.
"It was like a light bulb went off," she says. "I couldn't get it out of my head. I thought, 'Could this be?'"
Another doctor's appointment and tests confirmed Bradley had acute myeloid leukemia.
Bradley needed four rounds of chemotherapy and a bone marrow transplant.
"We wanted Charlie to feel like she was part of the decision-making process and wanted to prep her mentally," says Jennifer. "So we explained that Bradley's blood was sick and hers was healthy, and she said, 'Let's do it.' She's always been a giving kid."
The surgery took place in February at the Ann and Robert H. Lurie Children's Hospital of Chicago.
"What Charlie did for her brother and my wife and I was nothing short of amazing," Brian told to ABC News. "For us to be fortunate enough for Bradley to have a twin sister who's a perfect match; we were speechless. Not everyone is so lucky."
"We were almost at a loss for words as to how emotional it was."
The twins' transplant physician Dr. Jennifer Schneiderman said the procedure went as well as it could.
Bradley is now in remission and he and his sister are starting kindergarten.
"As a parent, you want to be a hero to your child," said Brian. "But our kids have been heroes to us. And this whole ordeal has brought the two of them even closer."