The parents of a U.K. toddler say it's "absolutely mind-blowing" to see their daughter, enrolled in a gene therapy trial, hear for the first time.

Opal Sandy was born into a world she could not hear. The British baby girl, now 18 months old, had a rare genetic condition called auditory neuropathy that interrupted nerve impulses that travel from the inner ear to the brain.

A British toddler born with a genetic condition resulting in deafness can now hear in one ear thanks to a pioneering new gene therapy treatment.

Opal Sandy, an 18-month-old from England, is the youngest child to come close to “normal hearing levels” in a gene therapy trial to aid her genetic deafness.

This therapy enabled the baby, who was born with a rare genetic condition that impaired ear function completely, to hear even faint sounds.

The gene therapy Opal received involved the transmission into her right ear of a working copy of the OTOF gene, delivered during surgery into the ear's cochlea via a harmless virus called AAV1. The baby girl was also fitted out with a cochlear implant in her left ear.

Regeneron Pharmaceuticals said on Wednesday its investigational gene therapy improved hearing to normal levels in a child within 24 weeks in an early to mid-stage study.

Regeneron (NASDAQ:REGN) announced Wednesday that a child who received its gene therapy candidate DB-OTO improved hearing to normal levels within 24 weeks in a Phase 1/2 trial for genetically linked deafness.