CD229, also known as Ly9 or SLAMF3, is a 120 kDa type I transmembrane glycoprotein belonging to the SLAM subgroup within the CD2 family. This protein is prominently expressed on T and B cells, thymocytes, and to a lesser extent on NK cells. The N-terminal Ig-like domain of CD229 facilitates homophilic binding between CD229 molecules.CD229/SLAMF3 serves as an autoligand receptor within the SLAM family, playing a crucial role in the regulation of immune cell activation and differentiation. It achieves this through homotypic or heterotypic interactions, which are essential for modulating immune responses.