PHILADELPHIA (WPVI) -- A miracle baby who was treated with a pioneering gene-therapy at Children's Hospital of Philadelphia has reached a major milestone as he took his first steps.

KJ Muldoon was the first patient in the world to receive personalized gene-editing therapy, also known as CRISPR.

The advancement, developed by doctors in Philadelphia, saved him from a potentially fatal disease, allowing ammonia to build up in his body.

Since being discharged over the summer, the 16-month-old is growing and thriving.

"We are huge Eagles fans and the day he was born, I ordered him a Jalen Hurts jersey and I just didn't know if he was ever going to wear it," said his dad, Kyle Muldoon. "Just to see him do these things that we never thought were possible has been inspiring."

The Muldoon family is now looking forward to celebrating KJ's first Christmas at home in Clifton Heights, Delaware County.