This rare immunodeficiency is typically associated with the absence or mutation of SAP.1, 2, 7, 9 SAP is expressed in leucocytes and links the kinase Fyn with SLAM family receptors in an atypical SH2–SH3 domain interaction.10, 11, 12 Absence of functional SAP in T cells and natural killer cells severely compromises cellular immunity.8, 9, 13, 14 SLAM family receptors manifest activating and inhibitory effects, and the term ‘switch motif’ is derived from the dual specificity of ITSMs for activating and inhibitory SH2‐domain‐containing proteins. This evidence concerns the gene SLAMF1 and immunodeficiency disease.