An experimental gene therapy was little Alissa’s only hope. Now, instead of certain death, she faces an uncertain future.

BOSTON — Thomas Feldborg and Daria Rokina set off nearly every afternoon to explore this city. They leave from their hotel in the Charlestown neighborhood, pushing their baby carriage, some days heading deep into downtown, others choosing a path along the Charles River.

Every few minutes, Rokina stops to peek inside and check on 16-month-old Alissa. She gently rubs Alissa’s cheek and coos a few soothing words, making sure the little girl is warm enough in her yellow snowsuit and adjusting her sparkly unicorn earmuffs.