Phelan-McDermid Syndrome is a genetic disorder that is associated with specific developmental and physical features. It results from the loss of a small amount of genetic material from Chromosome 22. This syndrome is also known as 22q13 Deletion Syndrome in reference to its chromosomal cause. The name is spoken, "22-q-one-three."
	Chromosomes are rod-like structures in the nucleus of your body cells. Normally, each of your body cells has 46 chromosomes, or 23 pair. One member of each chromosome pair is inherited from your mother and the other from your father. When chromosomes are studied in the laboratory, they are arranged by size, from the longest to the shortest. The shortest chromosome pair is number 22. Each chromosome is divided into two sections: a short arm (p) and a long arm (q). The loss (deletion) of a segment near the tip of the long arm (q) of chromosome 22, at the 13th band, results in Phelan-McDermid Syndrome.
Genes are small units of hereditary material that are found along the chromosomes. Genes are the instructions that tell our bodies how to develop and function properly. When a small piece of a chromosome is missing, the corresponding genes are also missing. When the instructions for proper development and function are incomplete, a genetic disorder may result. ProSAP2/SHANK3 is a gene that is important in brain development. ProSAP2/SHANK3 is at the 22q13 location and is lost when this segment of the chromosome is deleted. The loss of ProSAP2/SHANK3 explains the developmental features of Phelan-McDermid syndrome.