Detailed Instructions: In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.
Q: This structure is absent when the FOXN1 (read "fox-N-one") gene is deleted. Its epithelial cells express the AIRE gene and contain Hassall corpuscles. One syndrome in which it is missing is often associated with hypocalcemia due to low parathyroid activity and is known as DiGeorge syndrome. Patients with myasthenia gravis often have hyperplasia of this organ. Positive selection for cells that ultimately express either CD4 or CD8 occurs in this organ's cortex. Cancer development is often studied in mice lacking this organ because they fail to produce cells that recognize MHC-bound antigens. For 10 points, name this organ that shrinks as a person ages and that is the maturation site for T cells.
A:
Science