Brendan Maher did an excellent job in the article: Music, the Brain, and Williams Syndrome.1 In the study on absolute pitch, it should be noted that none of the five Williams participants were able to read musical notation. They were selected for the study because they were familiar with the names of musical notes and understood that those names are linked to musical tones. Both skills are rare in Williams people.
1.B. Maher, "Music, the brain, and Williams syndrome," The Scientist, 15:20, Nov. 26, 2001.