Most of the genetic alterations described in primary HLH impair NK- or T-cell functions—PRF1, UNC13D, STX11, STXBP2, and SH2D1A mutations causing familial HLH2, HLH3, HLH4, HLH5, and X-linked lymphoproliferative disease type 1 (XLP1), respectively. The gene discussed is UNC13D; the disease is X-linked lymphoproliferative syndrome.