FOXN1 is a forkhead box transcription factor that serves as a master regulator of thymic epithelial cell (TEC) development and function 1. It directs the differentiation of progenitor TECs into functional cortical and medullary TECs throughout fetal and postnatal thymic development, regulating genes essential for T-cell development including MHC Class II, DLL4, and CCL25 1. FOXN1 is critical for maintaining the three-dimensional thymic microstructure and vascularization necessary for immune tolerance and T-cell selection 2. Beyond the thymus, FOXN1 promotes epithelial differentiation in skin and hair follicles, influencing the developmental transition from scar-free to scar-forming wound healing 3. Loss-of-function FOXN1 mutations cause severe primary immunodeficiency (Nude/SCID syndrome) characterized by thymic aplasia and T-cell lymphopenia, representing the first SCID phenotype resulting from stromal rather than hematopoietic defects 1. During thymic aging, FOXN1 downregulation correlates with emergence of atypical TECs that form non-productive epithelial clusters and impair regenerative capacity 4. Age-related FOXN1 decline is a key component of thymic involution, driving immune senescence 5. FOXN1 mutations are associated with T-cell immunodeficiency, congenital alopecia, and nail dystrophy, highlighting its pleiotropic roles in immune and epithelial tissue development.