An American girl who was born with an extremely rare condition that kept her a toddler for life has died at the age of 20, her father told AFP.
Brooke Greenberg, whose rare condition was sometimes called syndrome X, raised scientists' hope of unlocking the secret to aging.
She died last week of a lung illness, Howard Greenberg said in a phone interview.
"We are going to remember her every day. She was a very, very, very special child," he said.
Brooke, who lived in Maryland with her parents and three sisters, stayed about the size of a two-year-old for life.
Only her hair and nails grew, according to an ABC news report that covered the family's story in 2009.
She had the mental capacity of a one year old and weighed about 15 pounds (seven kilograms).
She underwent a series of medical emergencies in her early years, including stomach ulcers, an apparent stroke, and an unexplained lethargy that caused her to sleep for two weeks.
She was diagnosed with a brain tumor and her family began preparing for her funeral, but then she suddenly opened her eyes and doctors could no longer find any tumor.
At age 16, she still had some baby teeth. She rode in an infant car seat and was often pushed around in a stroller during family shopping trips.
Brooke was unable to speak but communicated her wants and needs through vocalization and mannerisms, and her sisters said in the ABC news report that she sometimes rebelled the way a teenager would.
One of her physicians, Richard Walker, told ABC that he had seen minimal changes in Brooke's brain over time.
If scientists could decipher a genetic mutation that made Brooke the way she was, perhaps it could be tested in lab animals to help unlock the secrets of aging and mortality, Walker said.
Greenberg said his family was never aware of other children who had the same condition as their daughter.
"We have been told that she was one of 6.7 billion people," he told AFP. "She was a unique individual."