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Pallister-Killian syndrome
Background
Pallister-Killian syndrome (PKS) is a very rare chromosomal condition. A chromosome is a thread-like structure (divided into a short arm and a long arm) which is present in the nucleus of all body cells. The chromosomes carry the genes which are the instructions about how to make a new baby from a sperm and an egg. Usually individuals have twenty-three pairs of chromosomes (forty-six chromosomes in each cell). Individuals with PKS have an extra chromosome made up of material from the twelfth pair of chromosomes in some cells. This means there are forty-seven chromosomes in these cells rather than forty-six. The extra chromosome comprises two short arms of chromosome 12 (often referred to as isochromosome 12p). Because the affected person has extra genes, this causes the distinct features associated with PKS. The extra isochromosome 12 is not present in all body cells but only in part of them, with the other cells having a normal chromosome number. The condition is therefore described as a mosaic. It makes the diagnosis of PKS frequently difficult as the extra isochromosome is usually not present in blood cells. The extent to which an individual is affected by PKS depends in part upon the proportion of cells containing the isochromosome 12p throughout the body.
PKS affects both males and females equally. The condition is present at birth and the oldest known individuals with PKS are in their forties.
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