SASH3 (SAM and SH3 domain containing 3) is an X-linked adaptor protein that plays a critical role in lymphocyte signaling and immune system function 1. The protein functions as a signaling scaffold in T-cell receptor (TCR) signaling cascades, essential for proper T-cell proliferation, survival, and cell cycle progression 1. SASH3 contains sterile alpha motif (SAM) and Src homology-3 (SH3) domains that facilitate the organization of signaling complexes in lymphocytes 12. Mechanistically, SASH3 deficiency leads to impaired thymocyte survival, decreased T-cell proliferation, increased T-cell apoptosis, and defective cell cycle progression in response to mitogens 1. Disease relevance is significant, as deleterious SASH3 variants cause X-linked combined immunodeficiency with immune dysregulation, characterized by recurrent infections, autoimmune cytopenias, and Evans syndrome 13. Patients exhibit CD4+ T-cell lymphopenia, neutropenia, and B-cell/NK-cell deficiencies 1. Clinical significance includes the recent identification of SASH3 deficiency as a novel inborn error of immunity, with lentivirus-mediated gene therapy showing promise for correcting protein expression and cellular function in deficient cells 14.