Zhu Yangyang babbles away like a typical happy 3-year-old, calling out for "mama" and "papa" and accurately naming colors — a remarkable change considering he was completely deaf just months ago.

He is one of five children whose hearing was restored through a new gene therapy in a clinical trial led by Chinese and American researchers, offering new options for those born with a rare genetic mutation.

Yangyang's mother Chang Yiyi says she was moved to tears when she realized, around three weeks after the treatment last September, that Yangyang could hear her knocking on the door.