CD229, also known as Ly9 and SLAMF3, is a 120 kDa type I transmembrane glycoprotein in the SLAM subgroup of the CD2 family (1). Mature human CD229 consists of a 407 amino acid (aa) extracellular domain (ECD) with two Ig‑like V-set and two Ig‑like truncated C2-set domains. It also shows a 22 aa transmembrane segment, and a 179 aa cytoplasmic domain with two immunoreceptor tyrosine-based switch motifs ITSMs (2, 3). The ECD of human CD229 shares 57%‑59% aa sequence identity with mouse and rat CD229. Within the first two Ig‑like domains that are common to all SLAM proteins, human CD229 shares 24%‑39% aa sequence identity with human 2B4, BLAME, CD2F-10, CD84, CRACC, NTB-A, and SLAM. Alternate splicing generates two additional isoforms that lack the juxtamembrane Ig‑like domain or short cytoplasmic region (2). CD229 is expressed on T and B cells, thymocytes, and more weakly on NK cells (2‑6). Homophilic binding between CD229 molecules is mediated by the N-terminal Ig‑like domain (7). Human and mouse CD229 exhibit cross-species binding (7). Antigen stimulation of lymphocytes induces CD229 clustering to sites of T cell‑B cell contact (7). Two tyrosines in the cytoplasmic domain are required for association of CD229 with the adaptor proteins AP-2 (μ2 chain) and GRB-2 and both are required for CD229 internalization (8, 9). In addition, there are two ITSMs which mediate phosphorylation-dependent CD229 association with SAP and SHP-2 (10). These four tyrosine-containing motifs are intact in the described CD229 splice variants. CD229 knockout mice show minimally impaired immune responses, suggesting functional redundancy with other molecules (11).