<h4>Introduction</h4>X-linked lymphoproliferative disease type 1 (XLP1) is an inborn error of immunity (IEI) caused by pathogenic variants in the <i>SH2D1A</i> gene, leading to severe immune dysregulation, often triggered by Epstein-Barr virus (EBV) infection. The gene discussed is SH2D1A; the disease is X-linked lymphoproliferative syndrome.