Boys with mutations in the SH2D1A gene, which encodes SLAM-associated protein (SAP), have a complete absence of iNKT cells (18–20) and develop X-linked lymphoproliferative disease (XLP) (67, 68), a form of severe and often fatal IM typically triggered by EBV infection (66). Here, SH2D1A is linked to X-linked lymphoproliferative disease.