For X-linked gene defects it is normally expected that only male patients are affected, but there can be exceptions: 1 female out of 91 patients with agammaglobulinemia [with Turner syndrome (X,0)] had a monoallelic mutation in BTK, and 1 female out of 10 X-linked lymphoproliferative syndrome (XLP) patients had a mutation in SH2D1A (XLP1). The gene discussed is BTK; the disease is X-linked lymphoproliferative syndrome.