Studies of SAP-deficient (Sh2d1a-/-) mouse models have provided insight into this disease and revealed two additional phenotypes that have been confirmed in XLP1 patients: a lack of long-term humoral immunity associated with defective T cell help for germinal center (GC) formation [4], and the absence of invariant natural killer T (iNKT) cells [5–7]. Here, SH2D1A is linked to X-linked lymphoproliferative syndrome.