X-linked lymphoproliferative disease type 1 (XLP1) is a well-defined maternally-inherited immunodeficiency disease caused by a mutation of the SH2D1A gene, which encodes for signaling lymphocytic activation molecule (SLAM)-associated protein (SAP), leading to functional defects in natural killer (NK) cells, CD8+ T cells and natural killer T (NKT) cells (1). The gene discussed is SH2D1A; the disease is X-linked lymphoproliferative syndrome.