A gene therapy trial has successfully provided children who were born deaf the ability to hear.

The trial, conducted in Fudan, China, at the Fudan University’s Eye & ENT Hospital and by a team at the Massachusetts Eye and Ear Infirmary and Harvard Medical School, six children who suffer from a gene mutation that affects signals transmitted from the ear to the brain were treated.

The children, ages one through seven, participated in a 26-week trial in 2022 that examined the otoferlin gene. Five out of six of them showed improvement or recovery in hearing, the study found. Speech perception also improved in children who had hearing recovery.

By the end of the 26-week study, the three older children who had cochlear implants turned off could understand and respond to speech. Two were able to recognize speech in a noisy room and have a telephone conversation, the Harvard Gazette noted.

The Children’s Hospital of Philadelphia announced Tuesday that initial results of the trial in a patient indicate the treatment was successful.

“While the gene involved is quite rare, the milestone represents a breakthrough in the treatment of patients around the world with hearing loss caused by dozens of different genetic mutations and marks another innovative move for gene and cell therapy in a new area of medicine,” the hospital said in a statement.

The child in Philadelphia was 11, and born with an otoferlin gene-mediated hearing loss. He was born “profoundly deaf in both earns.” He underwent surgery on Oct. 4, 2023, his hearing has improved enough so he now “only had mild to moderate hearing loss in the ear that was treated.”

The children have undergone a surgical procedure where the gene therapy is placed into the inner ear using a device that partially lifts the eardrum. A single, small dose was given to the Philadelphia boy that contained copies of the normal gene.

More than 150 different genes have been implicated in hearing loss and about 1 in 500 newborn babies are affected by some form of genetic hearing loss, the hospital said.