Williams syndrome. Fluorescence in situ hybrid- isation (FISH) micrograph of human chromosomes showing Williams syndrome (Severe infantile idiopathic hypercalcaemia). This genetic disorder is caused by the deletion of a gene on chromosome 7. The chromosome marked 7 at lower right shows four dots (genes) at the corners of its lower half (green). The chromosome marked del(7) at upper left shows only one such green area. FISH is a technique which allows individual genes to be highlighted on the chromosomes. Williams syndrome leads to moderate mental retardation, and failure of the child to thrive.