On September 24, 2015, in a sixth floor room at Boston Children’s Hospital, 5-year-old Logan Lesselroth pressed the button that started the transfer of his newly harvested blood stem cells to his 3-year-old sister, Gianna.

“This,” Gianna told him, “is a gift from your body.”

The path to that moment and the stem cell transplant’s potential to cure Gianna of her relapsed leukemia was anything but straightforward. Logan has a genetic condition called medium-chain acyl-CoA dehydrogenase deficiency (MCADD), which makes it difficult for his body to convert sugar to energy. Would his metabolic disorder be passed to Gianna? Would the disorder make it too risky for Logan to have his stem cells harvested?

Diagnosed with acute lymphoblastic leukemia at the age of 4½ months, Gianna achieved a remission that lasted two years. In May 2015, the leukemia was back. With that, Mike and Marissa Lesselroth sought options for their daughter in their home state of Florida and beyond. “We talked to her doctors in Florida, and they agreed that coming to Dana-Farber/Boston Children’s was the best choice for Gianna because they offered a lot of treatment options for relapsed leukemia,” Mike says. …