The absence of an elastin gene from a chromosome is detected by the FISH technique. FISH is an acronym for the technical expression "fluorescence in situ hybridization." A blood sample is taken from your child and then treated with two specific colored markers that give off a "fluorescent" light when exposed to ultra-violet light. One of the markers attaches to each of the two copies of chromosome number seven in a cell. When both copies of chromosome possess the elastin gene, an additional fluorescence of another color is seen attached at another location to each of the two chromosome 7s. But, as is the case in over 95% of the Williams syndrome individuals tested, only one copy of chromosome seven, not two, will show the fluorescent spot for the elastin gene. For pictures illustrating chromosome seven and this test, check the article in the December, 1997 Scientific American magazine . (Lenhoff, et. al., pp. xx-xx). If the FISH test shows the elastin gene to be missing from one copy of chromosome seven, then the individual tested can be said to have Williams syndrome.