The disease was first characterized in 2006 by Rigaud et al. [2] in patients with characteristic symptoms of XLP, including hemophagocytic lymphohistiocytosis (HLH), hypogammaglobulinemia and cytopenia, but lacking mutations in the SLAM-associated protein (SAP) gene, which at the time was the known genetic defect to cause XLP (XLP-1). The gene discussed is SH2D1A; the disease is X-linked lymphoproliferative disease.