SH2D1A encodes SAP (SLAM-associated protein), a cytoplasmic adapter protein that regulates signaling through the SLAM family of receptors on T and NK cells 1. SAP functions by recruiting FYN kinase to phosphorylate and activate SLAM receptors, thereby enhancing NK cell-mediated cytotoxicity and conjugate formation with target cells 2. The protein is predominantly expressed in activated T cells and NK cells, where its expression is tightly regulated by Ets-1/Ets-2 transcription factors and rapid mRNA decay mechanisms involving the 3' UTR 3. Loss-of-function mutations in SH2D1A cause X-linked lymphoproliferative syndrome type 1 (XLP1), a severe immunodeficiency characterized by heightened susceptibility to Epstein-Barr virus (EBV) infection 4. XLP1 patients experience impaired CD8+ T cell proliferation and dysregulated interferon signaling upon EBV infection, contributing to hemophagocytic lymphohistiocytosis (HLH)—a life-threatening hyperinflammatory syndrome 5. SH2D1A variants account for approximately 17.8% of X-linked HLH cases in clinical cohorts 6. SAP expression is critical for proper T follicular helper (TFH) cell differentiation and germinal center formation, essential for adaptive immunity 2. These findings establish SH2D1A as a key regulator of immune homeostasis and EBV control.