The baby saved from a rare disease by a first-ever personalized gene fix has reached a big milestone, taking his first steps ahead of Christmas.

KJ Muldoon is walking and getting ready to celebrate the holiday season at home with his parents and three siblings.

KJ was born last year with a genetic disorder called carbamoyl-phosphate synthetase 1 deficiency. Also known as CPS1 deficiency, the metabolic condition affects only 1 in 1.3 million babies and often leads to life-threatening outcomes.

When he was 6 months old, doctors began giving KJ a groundbreaking new treatment -- a personalized CRISPR gene editing therapy at Children's Hospital of Philadelphia.

Doctors save baby's life with first-ever gene fix for deadly rare disease

The gene-editing therapy works like a "molecular spell-checker,” finding and fixing the specific genetic error causing his condition.

After three infusions and after spending the first 10 months of his life in a hospital, KJ was discharged and sent home in June.

Baby saved by gene-editing therapy 'graduates' from hospital, goes home

Since then, KJ has continued to grow and thrive with his family, celebrating his first birthday at home and taking part in one of the family’s favorite rituals -- cheering for their beloved Philadelphia Eagles.

Doctors hope new technology like CRISPR gene-editing therapies can be the key to treating more than 7,000 rare diseases, such as sickle cell disease, cystic fibrosis, Huntington’s disease and muscular dystrophy, that affect approximately 30 million people across the country.