Two clinical subtypes have been established: XLP type 1 (XLP1), caused by mutations in the SH2D1A gene, which encodes the signaling lymphocytic activation molecule (SLAM)-associated protein (SAP), and XLP type 2 (XLP2), associated with mutations in the XIAP gene, which encodes the X-linked inhibitor of apoptosis protein [2]. Here, SH2D1A is linked to X-linked lymphoproliferative syndrome.