CD57 (also known as HNK-1 or LEU-7) is an 110 kDa glycoprotein predominantly expressed on natural killer (NK) cells, terminally differentiated CD8+ T cells, some monocytes, and neural crest cells. In the immune system, CD57 expression is associated with cellular senescence and terminal differentiation, characterized by reduced proliferative capacity but enhanced cytotoxicity. In NK cells, CD57 marks a more mature phenotype with higher cytotoxicity and responsiveness to signals from CD16 and natural cytotoxicity receptors (NCRs). Additionally, CD57 plays a role in the nervous system, where it is involved in neural crest cell migration and invasion, and is associated with metastasis in melanoma. Clinically, CD57 is used as a diagnostic marker for neuroendocrine and nervous system tumors.