DiGeorge syndrome

DiGeorge syndrome A congenital disorder characterized by low blood calcium because of underdevelopment of the parathyroid glands needed to control calcium; immunodeficiency due to hypoplasia of the thymus (an organ behind the breastbone in which lymphocytes mature and multiply); and defects involving the outflow tracts from the heart. DiGeorge syndrome is caused by a very small deletion in chromosome band 22q11.2. The key gene that is lost is Tbx-1. Tbx-1 is a master control gene that regulates other genes required for the connection of the heart with the blood circulation. Tbx-1 also controls genes involved in the development of the parathyroid and thymus glands and the shape of the face. Also known as hypoplasia of the thymus and parathyroids and as third and fourth pharyngeal pouch syndrome.