This volume focuses on a challenging field in biomedicine: the genetic control of central immune tolerance. The thymus gland is a lymphoid organ implicated in T cells' maturation, differentiation, and selection. Its function is associated with the control of immune homeostasis in the body, establishing central immune tolerance, and preventing the onset of autoimmune diseases. This book focuses on thymus development, their cellular components and their respective function, and the peculiar gene expression profiling (transcriptome) found in the medullary thymic epithelial cells (mTECs) that are implicated in the self-representation in the thymus and the Autoimmune regulator (Aire) gene. Chapters also explore the mutations in the Aire gene, manifestation of autoimmune diseases, and the role of cell-cell interactions within the thymus with implications in the negative selection (elimination) of nascent autoreactive T cells in preventing aggressive autoimmunity. This new edition includes two new chapters devoted to the genome editing of the Aire gene through Crispr-Cas9 system, and thymic involution.All chapters have been updated to reflect the latest research in the field. .