Williams syndrome is caused by a  spontaneous genetic deletion. That means it's not caused by anything the parents may have done during pregnancy or at any other time. The deletion in Williams syndrome generally includes a small stretch of 26-28 adjacent genes on chromosome 7, including the elastin gene. Usually, when someone is born with Williams syndrome, he or she is the only member of the extended family that has the condition.  But, when a person with Williams syndrome has a child, there is a 50:50 chance that each child will inherit the condition. Here in Canada, a geneticist will often request that parents are tested to rule out this rare family connection.