In the months after the gene therapy infusion at Boston Children’s, her symptoms disappeared. But doctors had given her blood transfusions while she regrew her own red blood cells, so it was not clear if the absence of symptoms was because of the gene therapy or the transfusions.

As she recovered, Helen returned to her passion: dancing. One day, she came back from her school dance group and told her mother, “My legs hurt. It feels funny.” Ms. Cintron smiled. “That’s soreness,” she explained. Helen laughed. She had only known pain from sickle cell.

Helen was scheduled for her six-month checkup on Dec. 16. By then, all the transfused cells were gone, leaving only blood made by stem cells in her own marrow. The doctors would finally tell her whether the therapy was working.

The day before, she and her parents visited the New England Aquarium in Boston. She was able to stay outside on a cold, blustery day, watching one seal bully the others, barking and fighting. When Helen mentioned that her hands were cold, Ms. Cintron’s stomach clenched in fear. But it was just a normal thing to feel on a winter day.

The next morning, Dr. Esrick delivered the news. Helen’s total hemoglobin level was so high it was nearly normal — a level she had never before achieved even with blood transfusions. She had no signs of sickle cell disease.

“Now you are like me,” her father told her. “I jump in the pool, I run. Now you can do it, too!”

Her family, accustomed to constant vigilance, is only now getting used to normal life.

On Dec. 23, Helen and her mother flew to the family’s new home in Arizona.

Helen recently described her transformed outlook on Facebook.