 Nude mice with the very rare "nude" deficiency as a result of FOXN1 mutation are a strain of research mice as a model of T cell deficiency.The most common congenital cause of thymus-related immune deficiency results from the deletion of the 22nd chromosome, called DiGeorge syndrome.This results in a failure of development of the third and fourth pharyngeal pouches, resulting in failure of development of the thymus, and variable other associated problems, such as congenital heart disease, and abnormalities of mouth (such as cleft palate and cleft lip), failure of development of the parathyroid glands, and the presence of a fistula between the trachea and the oesophagus.