SAP (SLAM-associated protein) is a signaling molecule primarily expressed on immune cells, belonging to the SAP family proteins. It regulates the activation and inhibition of T cells and NK cells by binding to SLAM family receptors. SAP's structure mainly includes an SH2 domain, enabling it to bind to phosphotyrosine sites. The protein is located in lymphocytes and affects immune responses by influencing intercellular interactions and signaling pathways, such as the SLAM-SAP pathway. Specific expression of the SAP gene is crucial for normal immune function, and mutations in this gene are commonly associated with X-linked lymphoproliferative disease.