Microdeletion. Williams syndrome is caused by small loss (deletion) of genetic material from long arm of chromosome number 7.
William's Syndrome. Quoting from http://www. Williams-syndrome. Org/faq#1, "williams’s syndrome is caused by the spontaneous deletion of 26-28 genes on chromosome #7. The deletion occurs at the time of conception. In most families the child with williams’s syndrome is the only one to have the condition in his or her entire extended family.".
Missing genes. Rare condition caused by missing genes. Parents may not have a family history of the condition. Cause is random. Occurs in 1:8, 000 births.