Among the different SLAM receptors expressed by NK and CD8 T cells, NTB-A and 2B4 substantially contribute to the cytolytic defects of these effectors in XLP (Nakajima et al., 2000; Parolini et al., 2000; Tangye et al., 2000b; Bottino et al., 2001). The gene discussed is SLAMF6; the disease is X-linked lymphoproliferative disease.