CD56 is an isoform of the Neural Cell Adhesion Molecule (NCAM). CD56 is an adhesion molecule involved in intercellular homophilic adhesion and plays a role in outgrowth of neurites and the development of the nervous system. Furthermore CD56 is a marker for natural killer cells and found in various tumors.Several isoforms of NCAM have been identified: two transmembrane isoforms of 140 and 180 kD, a GPI-linked isoform of 120 kD which lacks a transmembrane domain and a fourth variant which is leading to the expression of a soluble form (sNCAM). Antibody 123C3 recognizes the transmembrane glycoprotein of 140 and 180 kD. At the international Workshop on SCLC antibody 123C3 has been categorized as cluster 1 antibody.All cells in small cell carcinomas and carcinoids of the lung are strongly positive for 123C3. In non small lung cell carcinomas, 123C3 staining has been associated with more advanced stage and a decreased survival after surgery. Positive staining with other tumors, include medullary thyroid carcinomas and some ovarian tumors. Furthermore, this antibody can be used to support diagnosis of lymphoma or to detect residual disease for cases of CD56 positive T/NK -cell lymphoma in which the neoplastic lymphoid cells are small and show minimal atypia, especially in small biopsies.