Williams syndrome is a rare genetic disorder that affects 1 in 20,000 live births. It was described by Dr.J.C.P. Williams in New Zealand in 1965. It is characterized by particular facial features, cardiac and neurological problems, unusually friendly and social demeanor and some other traits. Genetically, it is caused by the deletion of the long arm of chromosome 7, encompassing approximately 25 genes. 
Facial features help with the diagnosis and they include the following: 
Sunken nasal bridge
Puffiness around the eyes
Long upper lip length (philtrum)
Widely spaced teeth
Wide mouth
Prominent lower lip
Small Chin
Neurologic and behavioral traits include the following:
Impaired visuospatial abilities
Impaired problem solving
Preserved language skills
Love of music
Hyperacusis (hypersensitivity to loud noises)
Other physical traits include:
Hypercalcemia
Failure to thrive
For further information you may visit the website of Williams Syndrome Association.