Friendliness and fearlessness aside, people with Williams often have a number of physical ailments, including serious heart conditions. Such conditions cause many adults with Williams to survive only into their 50s.
Michelle Self's son, Alex, was diagnosed with Williams Syndrome when he was four weeks old.
"He had his heart surgery when he was a baby," Self told "20/20." "Last year they put two stents in and, during the process, he had a stroke. So I had a nine-year-old that had a stroke. We had to re-teach him to speak, to walk, to use his hands."
Now 11, Alex rides a bike, plays drums and piano, and attends a regular school.
Learn more about Alex, Kelley Martin and Williams Syndrome on "20/20" tomorrow at 10 p.m. ET.