FOXN1 and Aplasia of the thymus: Studies of nude mice (nu/nu) with a congenital lack of Foxn1 revealed that the absence of Foxn1 was responsible for athymia and for the characteristic hairless phenotype [9, 10]; however, the development of the thymic anlage was not impaired, suggesting that other factors upstream of Foxn1 may be responsible for organ fate [7, 11].