MSN (moesin) is a member of the ezrin-radixin-moesin (ERM) family that functions as a critical linker between the actin cytoskeleton and plasma membrane. The protein oscillates between resting and activated states, with C-terminal threonine phosphorylation triggering activation and subsequent F-actin interaction, leading to cytoskeletal rearrangements that regulate cell shape, membrane transport, and signal transduction 1. MSN plays particularly important roles in immune system function, regulating T and B-cell homeostasis, self-tolerance, and lymphocyte egress from lymphoid organs 23. Additionally, it participates in immunological synapse formation and modulates phagolysosomal biogenesis in macrophages 4. MSN deficiency causes severe combined immunodeficiency (Immunodeficiency 50), characterized by recurrent infections, lymphopenia, and autoimmune manifestations. A novel pathogenic mutation (c.68 A > G, p.N23S) was recently identified in a patient presenting with EBV infection, dermatomyositis-like symptoms, and eventual NK/T-cell lymphoma development 5. This mutation resulted in attenuated MSN protein expression, impaired T-cell proliferation and migration, and elevated autoantibody production, demonstrating MSN's essential role in immune cell function and disease prevention 5.