The signaling lymphocyte-activation molecule family member 3 (SLAMF3), also known as LY9, is a cell surface receptor that is expressed on T and B lymphocytes and belongs to the CD150/SLAMF1 receptor family. SLAMF3 was identified through a yeast two-hybrid screening in which SLAMF3 bound to the X-linked lymphoproliferative disease gene product SAP, suggesting that in activated T cells, SLAMF3 signaling can be triggered SAP. Its expression on the cell surface appears to be regulated via its interaction with the clathrin-associated adaptor complex 2 (AP-2). Despite its similarity to SLAMF1 in structure and interactions with SAP, SLAMF3-deficient mice do not exhibit phenotypic characteristics associated with SLAMF1- and SAP-deficient mice, suggesting that SLAMF3 plays other roles in T cell activation.