CD56 (NCAM1 / neural cell adhesion molecule 1) is a member of the immunoglobulin superfamily involved in hemophilic and heterophilic interactions between neurons and neurons and muscle. It is marker of neural lineage and is also expressed in natural killer cells where its expression acts as a reliable marker of activation. It is also present on subsets of CD4+ and CD8+ T lymphocytes. Staining is expected to be primarily cytoplasmic. In normal tissues, it is highly expressed in the brain and nerves, in neuroendocrine tissues, in natural killer (NK) lymphocytes, and is used to identify NK lymphomas. CD56 is often considered the most specific and sensitive of small cell neuroendocrine markers in the lung, and is useful in diagnosing small cell cancers with crush artifacts. References: Front Immunol 2017 8:892; Pruitt, 2011; Rekhtman, 2010; Kontogianni, 2005