The first (XLP-1) was defined by mutations in SH2D1A that encodes the SLAM adapter SAP (Coffey et al., 1998; Nichols et al., 1998; Sayos et al., 1998), while XLP-2 is caused by mutations in the gene encoding the X-linked inhibitor of apoptosis protein (XIAP) (Rigaud et al., 2006). Here, XIAP is linked to X-linked lymphoproliferative syndrome.