Williams Syndrome is caused by a very small chromosomal deletion on the long arm of chromosome 7. The deleted region includes the elastin gene (see figures A and B below), which encodes a protein that gives blood vessels the stretchiness and strength required to withstand a lifetime of use. The elastin protein is made only during embryo development and childhood, when blood vessels are formed. Because they lack the elastin protein, people with Williams Syndrome have disorders of the circulatory system, also known as vascular disorders.

The chromosomal deletion that causes Williams Syndrome is so small that it cannot be seen in a karyotype. However, the deletion can be observed using a special technique called fluorescent in situ hybridization, or FISH. This technique allows DNA sequences to be labeled with a fluorescent chemical (called a probe) that lights up when exposed to ultraviolet (UV) light. The Williams Syndrome deletion can be detected by labeling the elastin gene with a fluorescent probe. The gene will light up under a UV light only if it is present; a lack of signal indicates a deletion.

FISH (Fluorescent In Situ Hybridization)

When a deletion is too small to see it in a karyotype, what do you do?

Scientists have developed a more precise method than karyotype analysis to see the presence or absence of DNA sequences when looking at whole chromosomes. This technique is called fluorescent in situ hybridization, or FISH. In this technique, a chemical that fluoresces (shines brightly) is first attached to a molecule that can recognize specific DNA sequences. This labeled molecule is called a "probe." Chromosomes are prepared in a way that allows the probe to bind to matching DNA sequences on the chromosomes. This process is called hybridization. The labeled DNA probe that is bound to the chromosomes fluoresces when the chromosomes are exposed to UV light, showing the presence and location of the probe-bound DNA sequences. If you can see fluorescence from the probe, the gene is present. If not, the gene has been deleted.