XLP is due to a mutation at Xq25 that results in an abnormal immune response to EBV. This results in an unregulated overwhelming increase in macrophages, cytotoxic killer cells, and EBV-infected B cells [18]. It can be of two types, XLP1(SAP deficiency) and XLP2(XIAP deficiency). XLP1 has variable clinical presentations, including fatal HLH secondary to EBV [19] as seen in our patient; however, our patient tested negative for SAP mutation, and hence XLP1 was excluded. The gene discussed is XIAP; the disease is X-linked lymphoproliferative disease.