CD27, also known as TNFRSF7, is an approximately 55 kDa transmembrane protein in the TNF receptor superfamily. It functions as a co‑stimulatory molecule that supports lymphocyte activation and survival (1). Mature rat CD27 consists of a 163 amino acid (aa) extracellular domain (ECD) with three TNFR cysteine‑rich repeats, a 21 aa transmembrane segment, and a 47 aa cytoplasmic domain. Within the ECD, rat CD27 shares 66% and 85% aa sequence identity with human and mouse CD27, respectively. CD27 is expressed as a disulfide‑linked homodimer that carries N‑linked and O‑linked glycosylation (2, 3). Proteolytic cleavage of CD27 results in the shedding of a 28‑32 kDa fragment of the ECD (3). CD27 is weakly expressed on naïve T cells and NK cells and is up‑regulated upon cell activation (3, 4). It is also up‑regulated on activated germinal center B cells, plasma cells, and a subset of memory B cells (5, 6). CD27 binds to the transmembrane glycoprotein CD27 Ligand/CD70 which is expressed on activated B cells, activated T cells, and dendritic cells (1, 7, 8). This interaction contributes to the activation and survival of CD4 ⁺ helper T cells (7‑10), CD8 ⁺ effector T cells (11, 12), memory T cells (9), and NK cells (4, 13). Ligation of CD27 on B cells promotes germinal center formation and the expansion and affinity maturation of memory B cell responses (5, 14).