SH2D1A and X-linked lymphoproliferative syndrome: First described in 1975 by Purtilo et al. [197], X-linked lymphoproliferative syndrome type 1 (XLP1) is a rare X-linked immunodeficiency caused by germline variants in the gene SH2D1A with an incidence of 1 in every 1,000,000 – 2,000,000 male individuals [198].