CD56, also known as Neural-Cell Adhesion Molecule (NCAM), is a glycoprotein that participates in homophilic binding. It is found on the surface of neurons, glia, and skeletal muscle. CD56 is associated with various cellular processes such as cell-cell adhesion, neurite outgrowth, synaptic plasticity, as well as learning and memory.

In normal cells, CD56 antibody staining is observed in natural killer (NK) cells, activated T-cells, brain and cerebellum tissues, and neuroendocrine tissues. Tumors that show positive staining for CD56 include Myeloma, Myeloid Leukemia, Neuroendocrine tumors, Wilm's Tumor, Adult Neuroblastoma, NK/T cell Lymphomas, Pancreatic Acinar-cell Carcinoma, Pheochromocytoma, and Small-cell Lung Carcinoma. Additionally, some mesodermally-derived tumors like Rhabdomyosarcoma express CD56. However, Ewing's Sarcoma/PNET is negative for CD56.